Develop Reference

Cant Connect SQL Server to Visual Studio [duplicate]

I get the following error when trying to connect to SQL Server:
A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections. (provider: Named Pipes Provider, error: 40 - Could not open a connection to SQL Server)
This error is thrown when I try to configure my database to gridview in Visual Studio 2010. I'm at a loss as to how to debug this error.
How would you debug this error? What steps should I take in order to determine what is really going on here, in addition to the one mentioned in the error message?

I found the following techniques helpful:
Make sure your database engine is configured to accept remote connections:
Start > All Programs > SQL Server 2005 > Configuration Tools > SQL Server Surface Area Configuration
Click on Surface Area Configuration for Services and Connections
Select the instance that is having a problem > Database Engine > Remote Connections
Enable local and remote connections
Restart instance
You may need to create an exception on the firewall for the SQL Server instance and port you are using:
Start > Run > Firewall.cpl
Click on exceptions tab
Add sqlservr.exe (typically located in C:\Program Files (x86)\Microsoft SQL Server\MSSQL.x\MSSQL\Bin, check your installs for the actual folder path) and port (default is 1433)
Check your connection string as well
From FIX : ERROR : Could not open a connection to SQL Server:
Check if your SQL server services is up and running properly:
Go to All Programs > Microsoft SQL Server 2008 > Configuration Tools > SQL Server Configuration Manager > SQL Server Services
Check to make sure SQL Server service status is Running.
In addition, ensure that your remote server is in the same network. Run sqlcmd -L to ascertain if your server is included in your network list.
Enable TCP/IP in SQL Server Configuration
When two or more SQL Servers are connected across network they do all communication using TCP/IP. The default port of SQL Server installation is 1433. This port can be changed through SQL Server Configuration Manager. TCP/IP should be enabled for SQL Server to be connected.
Go to All Programs >> Microsoft SQL Server 2008 >> Configuration Tools >> SQL Server Configuration Manager >> Select TCP/IP
Right Click on TCP/IP >> Click on Enable
You must restart SQL Server Services for all the changes to take effect. Right click and go to menu properties to select location where default port of SQL Server can be changed.

I got Solution for me :
Open "SQL Server Configuration Manager"
Now Click on "SQL Server Network Configuration" and Click on "Protocols for Name"
Right Click on "TCP/IP" (make sure it is Enabled) Click on Properties
Now Select "IP Addresses" Tab -and- Go to the last entry "IP All"
Enter "TCP Port" 1433.
Now Restart "SQL Server .Name." using "services.msc" (winKey + r)
It Will Work...

Adding my heavily upvoted comment as an answer with screenshots.
I spent a lot of time on this, finally what worked for me is:
1) Open Sql Server Configuration Manager --> SQL Server Network configuration --> Protocols for <(INSTANCE)> --> TCP/IP (double click on it).
2) Select --> IP Addresses(Tab).
3) Go to the last entry IP All and mention TCP Port 1433.
4) Press Win+R and enter services.msc.
5) Now restart SQL Server <(INSTANCE)>.
After this, the problem got resolved!

I am solving that problem by opening Services then start running Sql Server (Sqlexpress) service.

This Error mainly came when the SQL Service is stopped.You need to Restart the service.To go to this window you have to search the Services like this-
Then Search for SQLSERVER(MSSQLSERVER) and Restart the service.
Hope this will work.

In case you're using the Express Edition:
You need to add \SQLEXPRESS after your server name
e.g. MY-SERVER\SQLEXPRESS

Press window + R (Run window Open) and in run window type "services.msc" and new services open find SQL SERVER with instance name in my case it's SQL SERVER(SQLEXPRESS) then start this service and try again it works for me Hope Its Works for You also.

I had the same error when I wanted to run my WinForms project (that includes working with a SQL Server database and that worked perfectly on my PC) on another PC. The problem was in Windows Firewall on my PC. I solved this by adding two rules. This is the whole procedure how to allow SQL Server through Windows Firewall:
Open "Run" and enter services.msc
Find the service for SQL Server (instance name) and SQL Server Browser. One at a time, right click, select "Properties", copy the path to exe file
Then open firewall.cpl, click allow an application or add rule, add the previously copied path (there is a procedure you need to follow), check Domain and Private, uncheck Public.
This is the YouTube link where you can see this procedure: Allow SQL Server through Windows Firewall

I encountered the same problem
In my case, I solved the problem in this way
Step 1: From start menu went to SQL server configuration manager
Step 2: Enabled TCP/IP
Step 3: Double clicked TCP/IP and went to IP Address last entry IP ALL and entered TCP Port 1433 then applied
Step 4 : then pressed win+r and wrote services.msc opened the Services then scrolled down then right clicked on SQL Server (MSSQLSERVER) choose restart
That resolved my problem.
Even if doing all of the above steps do not solve the problem then simply restart the PC then hopefully it will work.

This solution resolves both issues Network Error & service behind SQL server
I answered a similar question here, you need to stat the other open Run type-> services.msc - under services -> sort by stopped you will see a bunch of stopped SQL services Right click and start
To begin - there are 4 issues that could be causing the common LocalDb SqlExpress Sql Server connectivity errors SQL Network Interfaces, error: 50 - Local Database Runtime error occurred, before you begin you need to rename the v11 or v12 to (localdb)\mssqllocaldb
Troubleshooting Steps
You do not have the services running run this cmd, net start MSSQLSERVER or net start MSSQL$ instancename
You do not have the firewall ports here
configured
Your install has and issue/corrupt (the steps below help give you a nice clean start)
You did not rename the V11 or 12 to mssqllocaldb/SqlServer
I found that the simplest is to do the below - I have attached the pics and steps for help.
Resolution Steps:
First verify which instance you have installed, you can do this by checking the registry and by running cmd
cmd> Sqllocaldb.exe i
cmd> Sqllocaldb.exe s "whicheverVersionYouWantFromListBefore"
if this step fails, you can delete with option d cmd> Sqllocaldb.exe d "someDb"
cmd> Sqllocaldb.exe c "createSomeNewDbIfyouWantDb"
cmd> Sqllocaldb.exe start "createSomeNewDbIfyouWantDb"
Restart MSSql Server or Simply restart your machine!
Hold/Press to open the CMD, window + R and Type "services.msc"
Now look for sql server services, open find SQL SERVER with instance name
then ReStart this service and try again

If none of the above solutions work (nothing worked for me) then just RESTART your computer and you will be able to connect to your sql server (localhost).

After doing everything mentioned here:
http://blog.sqlauthority.com/2009/05/21/sql-server-fix-error-provider-named-pipes-provider-error-40-could-not-open-a-connection-to-sql-server-microsoft-sql-server-error/
Still did not work for me.
Steps worked for me:
Start > Run > cmd > sqlcmd -L
It will prompt you the server name. Make sure this server name is same as the one you are trying to get connected to in CONNECT TO SERVER box of SQL management studio.
I made this silly mistake I keep using MSSQLSERVER rather using this server name.
Hope this helps for the people who make silly mistake like me.
Thanks.

I am using SQL Server 2016 and Window 10.
First thing is to allow remote connection to SQL Server.
What I did is to type sqlservermanager13.msc at start menu in order to open the SQL Server Configuration Manager. Make sure the TCP/IP status is enabled.
Check your TCP port number by double click TCP/IP protocol name. Usually it is 1433 by default.
The following procedures configure the Windows Firewall by using the Windows Firewall with Advanced Security Microsoft Management Console (MMC) snap-in. The Windows Firewall with Advanced Security only configures the current profile.
To open a port in the Windows firewall for TCP access
On the Start menu, click Run, type WF.msc, and then click OK.
In the Windows Firewall with Advanced Security, in the left pane, right-click Inbound Rules, and then click New Rule in the action pane.
In the Rule Type dialog box, select Port, and then click Next.
In the Protocol and Ports dialog box, select TCP. Select Specific local ports, and then type the port number of the instance of the
Database Engine, such as 1433 for the default instance. Click Next.
In the Action dialog box, select Allow the connection, and then click Next.
In the Profile dialog box, select any profiles that describe the computer connection environment when you want to connect to the
Database Engine, and then click Next.
In the Name dialog box, type a name and description for this rule, and then click Finish.
Another thing to configure.
To open access to SQL Server when using dynamic ports
On the Start menu, click Run, type WF.msc, and then click OK.
In the Windows Firewall with Advanced Security, in the left pane,
right-click Inbound Rules, and then click New Rule in the action
pane.
In the Rule Type dialog box, select Program, and then click Next.
In the Program dialog box, select This program path. Click Browse,
and navigate to the instance of SQL Server that you want to access
through the firewall, and then click Open. By default, SQL Server is
at C:\Program Files\Microsoft SQL
Server\MSSQL13.MSSQLSERVER\MSSQL\Binn\Sqlservr.exe. Click Next.
In the Action dialog box, select Allow the connection, and then
click Next.
In the Profile dialog box, select any profiles that describe the
computer connection environment when you want to connect to the
Database Engine, and then click Next.
In the Name dialog box, type a name and description for this rule,
and then click Finish.
Have a look at Microsoft doucmentation
Configure a Windows Firewall for Database Engine Access

You can test the following methods.
a
Check the connection string of the project.
b
Go to services and restart SQLServer Instance.
c
Open 'SQLServer Configuration Manager'
In the left panel select 'SQLServer Network Configuration' and expanding it
Select 'Protocols for MSSQLServer'
In the right panel dbl click on 'TCP/IP'
In the 'Protocol' tab set the 'Enabled' to 'Yes'
In the 'IP Addresses' tab scroll to down
In the 'IPAll' set 'TCP Port' to 1433
d
Open 'Firewall with advanced security'
In the right tab select 'Inbound Rules'
In the middle tab find the record that 'local Port' is 1433, If you can't found it try to created it with following levels
In the Start menu, click Run, type 'WF.msc', and then click OK
In the left panel click the 'Windows Firewall with Advanced Security'
In the right panel right-click 'Inbound Rules', and then click 'New Rule'
In the Rule Type dialog box, select 'Port', and then click Next
In the Protocol and Ports dialog box, select 'TCP', and select 'Specific Local Ports', and then type the port number 1433, Click Next
In the Action dialog box, select Allow the connection, and then click Next
In the 'Profile' dialog box, checking the Domain, Private and Public, Then click Next
In the 'Name' dialog box, type 'SQL 1433 Port' and for a description write description for own. Then click Finish
Then in the middle tab double click the found item(Instance) or created item of 'SQL 1433 Port' name by you.
Select 'Scope' tab in opened dialog box(SQL Server Properties)
In the Local PC Go to google.com in your Browser and search 'My IP'.
then copy of your 'IP'
Go to the remote server and in the 'SQL Server Properties' dialog box of 'Scope' tab, in the 'Remote IP Address' select the 'These IP Addresses' option and click 'Add' button
In the opened dialog box(IP Address) select 'This IP Address or Subnet' option and paste your 'IP', click OK button.

I have to run SQL Server Browser service into SQL Server Configuration Manager.
Installation can't discover newly created service without this.

I tried all the other answers on this question and some if not all probably played a part in getting this working for me, but I still couldn't connect to the DB remotely. I was using a SQL Server on an Azure VM.
I eventually remembered that the VM has endpoints that are controlled by the Azure account proxy, so I went on to the Azure Portal and added 1433 as an available endpoint and I could connect to my SQL instance.
Hope this helps someone who has tried all the other answers and is still having no luck!

I was experiencing the same problem and the problem was that I hade several projects in the solution (Weband Droid) and even though Default project was choosen in the Package Manager Console it used the connection string from the Droid project:
PM> update-database -Verbose
Using StartUp project 'Droid'. <-- DROID
Using NuGet project 'Web'. <-- WEB
Specify the '-Verbose' flag to view the SQL statements being applied to the target database.
<!-- BAD TARGET DATABASE -->
Target database is: 'DefaultConnection' (DataSource: .\SQLEXPRESS, Provider: System.Data.SqlClient, Origin: Convention).
System.Data.SqlClient.SqlException (0x80131904): A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections. (provider: SQL Network Interfaces, error: 26 - Error Locating Server/Instance Specified)
[REMOVED TEXT]
ClientConnectionId:00000000-0000-0000-0000-000000000000
Error Number:-1,State:0,Class:20
A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections. (provider: SQL Network Interfaces, error: 26 - Error Locating Server/Instance Specified)
After setting the Startup Project to Web and the Default Project in Package Manger Console I got it to work.

You may check service status of MS SQL Server 2014. In Windows 7 you can do that by:
Go to search and Type "SQL Server 2014 Configuration Manager
Then click on "SQL Server Service" on left menu
Check the instance of SQL Server service status if it is stopped or running
If it has stopped, please change the status to running and log in to SQL Server Management Studio 2014

Just restart SQL Server (MSSQLSERVER) service.

While the above solutions should work in 90% of the cases, but if you are still reading this answer!!! You are probably trying to connect to a different server than intended. It may be due to a configuration file pointing to a different SQL server than the actual server you think you are trying to connecting to.
Happened to me atleast.

When I experienced this error in Visual Studio,
“A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections. (provider: Named Pipes Provider, error: 40 - Could not open a connection to SQL Server)”
...it was during the execution of the following C# code, which was attempting to obtain my SQL Server data to display it in a grid. The break occurred exactly on the line that says connect.Open():
using (var connect = Connections.mySqlConnection)
{
const string query = "SELECT Name, Birthdate, Narrative FROM Friends";
using (var command = new SqlCommand(query, connect))
{
connect.Open();
using (var dr = command.ExecuteReader())
{
while (dr.Read())
{
// blah
}
}
}
}
It was inexplicable because the SQL query was very simple, I had the right connection string, and the database server was available. I decided to run the actual SQL query manually myself in SQL Management Studio and it ran just fine and yielded several records. But one thing stood out in the query results: there was some improperly encoded HTML text inside a varchar(max) type field within the Friends table (specifically, some encoded comment symbols of the sort <!-- lodged within the "Narrative" column's data). The suspect data row looked like this:
Name Birthdate Narrative
==== ========= ==============
Fred 21-Oct-79 <!--HTML Comment -->Once upon a time...
Notice the encoded HTML symbol "<", which stood for a "<" character. Somehow that made its way into the database and my C# code could not pick it up! It failed everytime right at the connect.Open() line! After I manually edited that one row of data in the database table Friends and put in the decoded "<" character instead, everything worked! Here's what that row should have looked like:
Name Birthdate Narrative
==== ========= ==============
Fred 21-Oct-79 <!--HTML Comment -->Once upon a time...
I edited the one bad row I had by using this simple UPDATE statement below. But if you had several offending rows of encoded HTML, you might need a more elaborate UPDATE statement that uses the REPLACE function:
UPDATE Friends SET Narrative = '<!--HTML Comment -->Once upon a time...' WHERE Narrative LIKE '&lt%'
So, the moral of the story is (at least in my case), sanitize your HTML content before storing it in the database and you won't get this cryptic SQL Server error in the first place! (Uh, properly sanitizing/decoding your HTML content is the subject of another discussion worthy of a separate StackOverflow search if you need more information!)

Why this error is so boresome and noisy, just because it can occur in varied situation.
I have done all approchs above here, and still being sucked. So make sure u have done the same as me before browsing downward.
Maybe I am not able to fix ur situation instantly, but I can point out a direction or thinking to u(The one who finally slide down here). I have started to ponder the error of my running program occurring after I made sure that the instance name is clearly right and set my database to allow remote control following the methods above. After then, I suspected something wrong happening in my code snippet of SQL connection.
Solution of my problem:
Check my sqlconnection function
Click to see its configuration
New a connection
Select ur server name
It works for me with pondering what exactly happen in the process of connection.Hope my thinking will lead u to kill ur error.

Try adding a , and a port number (as in ,1433) to the end of your connection string.

Summary
To fix this issue encountered while running local app vs remote database, use SQL Server Configuration Manager to add an alias for the remote database.
Details
I had run into this problem recently when transitioning from a Windows 7 to a Windows 10 laptop. I was running a local development and runtime environment accessing our Dev database on a remote server. We access the Dev database through a server alias setup through SQL Server Client Network Utility (cliconfg.exe). After confirming that the alias was correctly setup in both the 64 and 32 bit versions of the utility and that the database server was accessible from the new laptop via SSMS, I still got the error seen by the OP (not the OP's IP address, of course).
It was necessary to use SQL Server Configuration Manager to add an alias for the remote Dev database server. Fixed things right up.

I moved from a work laptop on Windows 7 to a work laptop on Windows 10.
I had been successfully using SSMS2016 on Windows 7.
The same issue was applied using SSMS2012 or SSMS2016.
My access to the 10 SQL servers using windows authentication was still the same. I could test this from another server.
However, 2 of the 10 servers would not connect from my laptop.
Both were ms SQL server 9 but I could connect to another SQL server 9 databases.
The solution was to add a firewall rule (using Windows Firewall with Advanced Security).
Create an Incoming rule for each SSMS
eg C:\Program Files (x86)\Microsoft SQL Server\130\Tools\Binn\ManagementStudio\Ssms.exe
I'm not a network expert so I've not included the details but hopefully, it will point you in the right direction.
Error msg (pre firewall rule)
"A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections. (provider: SQL Network Interfaces, error: 26 - Error Locating Server/Instance Specified) (.Net SqlClient Data Provider)"

If you're encountering this while debugging in Visual Studio, make sure that the project build path points to a local drive, or follow these steps to grant permissions to the network folder.

My issue started when I tried to change the server from IIS Express to Local IIS (while using LocalDB).
I was using LocalDB (for dev purposes), and when I went to revert from Local IIS to IIS Express, Visual Studio had switched my data source from Data Source=(LocalDb)\MSSQLLocalDB to Data Source=.\SQLEXPRESS
Incorrect connection string
<add name="DefaultConnection" connectionString="Data Source=.\SQLEXPRESS;AttachDbFilename=|DataDirectory|\SurveyTestsDB.mdf;Initial Catalog=SurveyTestsDB;Integrated Security=True;User Instance=True"
providerName="System.Data.SqlClient" />
Correct connection string
<add name="DefaultConnection" connectionString="Data Source=(LocalDb)\MSSQLLocalDB;AttachDbFilename=|DataDirectory|\SurveyTestsDB.mdf;Initial Catalog=SurveyTestsDB;Integrated Security=True" providerName="System.Data.SqlClient" />
Hope this helps someone out there.

Along with trying everything as suggested by Teo Chuen Wei Bryan, make sure you are also referring to the correct Server/Instance name in the connection string.
If you are using the short form of host name/Instance on the database server or in the web.config file, make sure you use the fully qualified domain name(FQDN)/Instance
Also, to test connectivity from a server where SQL server client is NOT present,
--> create a text file and change its file extension to .udl
--> Right click the file and you can see connection tab.
--> Input server name and log on information to test connection to the database server.
Hope this helps.

If you suddenly encounter this error say in a production environment and nothing has changed, try the following 4 items in the order below to see if it gets fixed.
restart the sql server service.
restart the service (say IIS) that is calling into sql server. (the problem is probably here if the time between the start of the service call to SQL server and the time you end up getting the response error is super short (about one or two second).
restart the server sql server is on.
restart the server the calling service is on.

Xml tag arrangement in Web.config is important
First
<configSections>
<!-- For more information on Entity Framework configuration, visit http://go.microsoft.com/fwlink/?LinkID=237468 -->
<section name="entityFramework" type="System.Data.Entity.Internal.ConfigFile.EntityFrameworkSection, EntityFramework, Version=6.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" requirePermission="false" />
</configSections>
After
<connectionStrings>
<add name="SqlConnectionString" connectionString="Data Source=.; Initial Catalog=TestDB; Trusted_Connection=True;" providerName="System.Data.SqlClient" />
</connectionStrings>

UWP the application cannot communicate with the remote instance SQL Server

I am currently implementing a UWP program for an existing database. I took this article as a basis.
https://learn.microsoft.com/en-us/windows/uwp/data-access/sql-server-databases
When running, I get an exception:
System.Data.SqlClient.SqlException" в System.Data.SqlClient.dll
Exception:
A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections. (provider: TCP Provider, error: 25 - Connection string is not valid)
This seemed strange to me, since a WPF application with the same connection string works fine. Next, I decided to install a database instance on my computer and export some of the data to a local database. To my surprise, after replacing the connection string from:
Data Source=sqlserver\sqlexpress
to
Data Source=.\sqlexpress
UWP program read this from the database on my computer.
I also read this article to make sure that the remote database is configured correctly. https://www.lansweeper.com/knowledgebase/a-network-related-or-instance-specific-error-occurred/
As I said, WPF programs work with this database perfectly. What are the ways to check why the UWP doesn't want to communicate with the database?
to make it clear whether the remote server is available in principle, I apply the output of the TNC command:
tnc sqlserver -p 1433 -I detailed
ComputerName : sqlserver
RemoteAddress : 192.168.10.11
RemotePort : 1433
NameResolutionResults : 192.168.10.11
MatchingIPsecRules :
NetworkIsolationContext : Private Network
InterfaceAlias : Ethernet
SourceAddress : 192.168.10.20
NetRoute (NextHop) : 0.0.0.0
TcpTestSucceeded : True
Connection string look like:
private string connectionString = #"Data Source=sqlserver\sqlexpress;Initial Catalog=TestCatalog;User ID=*;Password=*";
The same connection string is used in VS. VS shows the contents of the table without problems. I can also delete or add new lines in the VS editor.
I'm talking about a remote database. In VS server Explorer I have a Data Connection where I am connected to a remote database. I can safely edit remote database data in VS (add, delete and update). I see the connection string and originally used it as the connection string in UWP app.

You have already taken a look at this microsoft link here. Long story short, have you followed the proposal below?
Trouble connecting to your database?
In most cases, some aspect of the SQL Server configuration needs to be changed. If you're able to connect to your database from another type of desktop application such as a Windows Forms or WPF application, ensure that you've enabled TCP/IP for SQL Server. You can do that in the Computer Management console.
Check to see if you have followed the rest of the solution properly. You might need to enable the tcp port from the window firewall.
Go to firewall -> Inbound Rules -> Add TCP:1433 in the rule.
Also since the error is about the connection string, try to use only the server name/ ip/ or hostname, without the server instance.
You can also add the port number and see if this works for you. When you add it, sql server ignores the instance name (as different instances require different ports to work, so it can work as a unique identifier).
Edit: How to create a valid connection string:
Look at this answer here: How to get the connection String from a database

sql connection issue: The server was not found or was not accessible [duplicate]

I get the following error when trying to connect to SQL Server:
A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections. (provider: Named Pipes Provider, error: 40 - Could not open a connection to SQL Server)
This error is thrown when I try to configure my database to gridview in Visual Studio 2010. I'm at a loss as to how to debug this error.
How would you debug this error? What steps should I take in order to determine what is really going on here, in addition to the one mentioned in the error message?

I found the following techniques helpful:
Make sure your database engine is configured to accept remote connections:
Start > All Programs > SQL Server 2005 > Configuration Tools > SQL Server Surface Area Configuration
Click on Surface Area Configuration for Services and Connections
Select the instance that is having a problem > Database Engine > Remote Connections
Enable local and remote connections
Restart instance
You may need to create an exception on the firewall for the SQL Server instance and port you are using:
Start > Run > Firewall.cpl
Click on exceptions tab
Add sqlservr.exe (typically located in C:\Program Files (x86)\Microsoft SQL Server\MSSQL.x\MSSQL\Bin, check your installs for the actual folder path) and port (default is 1433)
Check your connection string as well
From FIX : ERROR : Could not open a connection to SQL Server:
Check if your SQL server services is up and running properly:
Go to All Programs > Microsoft SQL Server 2008 > Configuration Tools > SQL Server Configuration Manager > SQL Server Services
Check to make sure SQL Server service status is Running.
In addition, ensure that your remote server is in the same network. Run sqlcmd -L to ascertain if your server is included in your network list.
Enable TCP/IP in SQL Server Configuration
When two or more SQL Servers are connected across network they do all communication using TCP/IP. The default port of SQL Server installation is 1433. This port can be changed through SQL Server Configuration Manager. TCP/IP should be enabled for SQL Server to be connected.
Go to All Programs >> Microsoft SQL Server 2008 >> Configuration Tools >> SQL Server Configuration Manager >> Select TCP/IP
Right Click on TCP/IP >> Click on Enable
You must restart SQL Server Services for all the changes to take effect. Right click and go to menu properties to select location where default port of SQL Server can be changed.

I got Solution for me :
Open "SQL Server Configuration Manager"
Now Click on "SQL Server Network Configuration" and Click on "Protocols for Name"
Right Click on "TCP/IP" (make sure it is Enabled) Click on Properties
Now Select "IP Addresses" Tab -and- Go to the last entry "IP All"
Enter "TCP Port" 1433.
Now Restart "SQL Server .Name." using "services.msc" (winKey + r)
It Will Work...

Adding my heavily upvoted comment as an answer with screenshots.
I spent a lot of time on this, finally what worked for me is:
1) Open Sql Server Configuration Manager --> SQL Server Network configuration --> Protocols for <(INSTANCE)> --> TCP/IP (double click on it).
2) Select --> IP Addresses(Tab).
3) Go to the last entry IP All and mention TCP Port 1433.
4) Press Win+R and enter services.msc.
5) Now restart SQL Server <(INSTANCE)>.
After this, the problem got resolved!

I am solving that problem by opening Services then start running Sql Server (Sqlexpress) service.

This Error mainly came when the SQL Service is stopped.You need to Restart the service.To go to this window you have to search the Services like this-
Then Search for SQLSERVER(MSSQLSERVER) and Restart the service.
Hope this will work.

In case you're using the Express Edition:
You need to add \SQLEXPRESS after your server name
e.g. MY-SERVER\SQLEXPRESS

Press window + R (Run window Open) and in run window type "services.msc" and new services open find SQL SERVER with instance name in my case it's SQL SERVER(SQLEXPRESS) then start this service and try again it works for me Hope Its Works for You also.

I had the same error when I wanted to run my WinForms project (that includes working with a SQL Server database and that worked perfectly on my PC) on another PC. The problem was in Windows Firewall on my PC. I solved this by adding two rules. This is the whole procedure how to allow SQL Server through Windows Firewall:
Open "Run" and enter services.msc
Find the service for SQL Server (instance name) and SQL Server Browser. One at a time, right click, select "Properties", copy the path to exe file
Then open firewall.cpl, click allow an application or add rule, add the previously copied path (there is a procedure you need to follow), check Domain and Private, uncheck Public.
This is the YouTube link where you can see this procedure: Allow SQL Server through Windows Firewall

I encountered the same problem
In my case, I solved the problem in this way
Step 1: From start menu went to SQL server configuration manager
Step 2: Enabled TCP/IP
Step 3: Double clicked TCP/IP and went to IP Address last entry IP ALL and entered TCP Port 1433 then applied
Step 4 : then pressed win+r and wrote services.msc opened the Services then scrolled down then right clicked on SQL Server (MSSQLSERVER) choose restart
That resolved my problem.
Even if doing all of the above steps do not solve the problem then simply restart the PC then hopefully it will work.

This solution resolves both issues Network Error & service behind SQL server
I answered a similar question here, you need to stat the other open Run type-> services.msc - under services -> sort by stopped you will see a bunch of stopped SQL services Right click and start
To begin - there are 4 issues that could be causing the common LocalDb SqlExpress Sql Server connectivity errors SQL Network Interfaces, error: 50 - Local Database Runtime error occurred, before you begin you need to rename the v11 or v12 to (localdb)\mssqllocaldb
Troubleshooting Steps
You do not have the services running run this cmd, net start MSSQLSERVER or net start MSSQL$ instancename
You do not have the firewall ports here
configured
Your install has and issue/corrupt (the steps below help give you a nice clean start)
You did not rename the V11 or 12 to mssqllocaldb/SqlServer
I found that the simplest is to do the below - I have attached the pics and steps for help.
Resolution Steps:
First verify which instance you have installed, you can do this by checking the registry and by running cmd
cmd> Sqllocaldb.exe i
cmd> Sqllocaldb.exe s "whicheverVersionYouWantFromListBefore"
if this step fails, you can delete with option d cmd> Sqllocaldb.exe d "someDb"
cmd> Sqllocaldb.exe c "createSomeNewDbIfyouWantDb"
cmd> Sqllocaldb.exe start "createSomeNewDbIfyouWantDb"
Restart MSSql Server or Simply restart your machine!
Hold/Press to open the CMD, window + R and Type "services.msc"
Now look for sql server services, open find SQL SERVER with instance name
then ReStart this service and try again

If none of the above solutions work (nothing worked for me) then just RESTART your computer and you will be able to connect to your sql server (localhost).

After doing everything mentioned here:
http://blog.sqlauthority.com/2009/05/21/sql-server-fix-error-provider-named-pipes-provider-error-40-could-not-open-a-connection-to-sql-server-microsoft-sql-server-error/
Still did not work for me.
Steps worked for me:
Start > Run > cmd > sqlcmd -L
It will prompt you the server name. Make sure this server name is same as the one you are trying to get connected to in CONNECT TO SERVER box of SQL management studio.
I made this silly mistake I keep using MSSQLSERVER rather using this server name.
Hope this helps for the people who make silly mistake like me.
Thanks.

I am using SQL Server 2016 and Window 10.
First thing is to allow remote connection to SQL Server.
What I did is to type sqlservermanager13.msc at start menu in order to open the SQL Server Configuration Manager. Make sure the TCP/IP status is enabled.
Check your TCP port number by double click TCP/IP protocol name. Usually it is 1433 by default.
The following procedures configure the Windows Firewall by using the Windows Firewall with Advanced Security Microsoft Management Console (MMC) snap-in. The Windows Firewall with Advanced Security only configures the current profile.
To open a port in the Windows firewall for TCP access
On the Start menu, click Run, type WF.msc, and then click OK.
In the Windows Firewall with Advanced Security, in the left pane, right-click Inbound Rules, and then click New Rule in the action pane.
In the Rule Type dialog box, select Port, and then click Next.
In the Protocol and Ports dialog box, select TCP. Select Specific local ports, and then type the port number of the instance of the
Database Engine, such as 1433 for the default instance. Click Next.
In the Action dialog box, select Allow the connection, and then click Next.
In the Profile dialog box, select any profiles that describe the computer connection environment when you want to connect to the
Database Engine, and then click Next.
In the Name dialog box, type a name and description for this rule, and then click Finish.
Another thing to configure.
To open access to SQL Server when using dynamic ports
On the Start menu, click Run, type WF.msc, and then click OK.
In the Windows Firewall with Advanced Security, in the left pane,
right-click Inbound Rules, and then click New Rule in the action
pane.
In the Rule Type dialog box, select Program, and then click Next.
In the Program dialog box, select This program path. Click Browse,
and navigate to the instance of SQL Server that you want to access
through the firewall, and then click Open. By default, SQL Server is
at C:\Program Files\Microsoft SQL
Server\MSSQL13.MSSQLSERVER\MSSQL\Binn\Sqlservr.exe. Click Next.
In the Action dialog box, select Allow the connection, and then
click Next.
In the Profile dialog box, select any profiles that describe the
computer connection environment when you want to connect to the
Database Engine, and then click Next.
In the Name dialog box, type a name and description for this rule,
and then click Finish.
Have a look at Microsoft doucmentation
Configure a Windows Firewall for Database Engine Access

You can test the following methods.
a
Check the connection string of the project.
b
Go to services and restart SQLServer Instance.
c
Open 'SQLServer Configuration Manager'
In the left panel select 'SQLServer Network Configuration' and expanding it
Select 'Protocols for MSSQLServer'
In the right panel dbl click on 'TCP/IP'
In the 'Protocol' tab set the 'Enabled' to 'Yes'
In the 'IP Addresses' tab scroll to down
In the 'IPAll' set 'TCP Port' to 1433
d
Open 'Firewall with advanced security'
In the right tab select 'Inbound Rules'
In the middle tab find the record that 'local Port' is 1433, If you can't found it try to created it with following levels
In the Start menu, click Run, type 'WF.msc', and then click OK
In the left panel click the 'Windows Firewall with Advanced Security'
In the right panel right-click 'Inbound Rules', and then click 'New Rule'
In the Rule Type dialog box, select 'Port', and then click Next
In the Protocol and Ports dialog box, select 'TCP', and select 'Specific Local Ports', and then type the port number 1433, Click Next
In the Action dialog box, select Allow the connection, and then click Next
In the 'Profile' dialog box, checking the Domain, Private and Public, Then click Next
In the 'Name' dialog box, type 'SQL 1433 Port' and for a description write description for own. Then click Finish
Then in the middle tab double click the found item(Instance) or created item of 'SQL 1433 Port' name by you.
Select 'Scope' tab in opened dialog box(SQL Server Properties)
In the Local PC Go to google.com in your Browser and search 'My IP'.
then copy of your 'IP'
Go to the remote server and in the 'SQL Server Properties' dialog box of 'Scope' tab, in the 'Remote IP Address' select the 'These IP Addresses' option and click 'Add' button
In the opened dialog box(IP Address) select 'This IP Address or Subnet' option and paste your 'IP', click OK button.

I have to run SQL Server Browser service into SQL Server Configuration Manager.
Installation can't discover newly created service without this.

I tried all the other answers on this question and some if not all probably played a part in getting this working for me, but I still couldn't connect to the DB remotely. I was using a SQL Server on an Azure VM.
I eventually remembered that the VM has endpoints that are controlled by the Azure account proxy, so I went on to the Azure Portal and added 1433 as an available endpoint and I could connect to my SQL instance.
Hope this helps someone who has tried all the other answers and is still having no luck!

I was experiencing the same problem and the problem was that I hade several projects in the solution (Weband Droid) and even though Default project was choosen in the Package Manager Console it used the connection string from the Droid project:
PM> update-database -Verbose
Using StartUp project 'Droid'. <-- DROID
Using NuGet project 'Web'. <-- WEB
Specify the '-Verbose' flag to view the SQL statements being applied to the target database.
<!-- BAD TARGET DATABASE -->
Target database is: 'DefaultConnection' (DataSource: .\SQLEXPRESS, Provider: System.Data.SqlClient, Origin: Convention).
System.Data.SqlClient.SqlException (0x80131904): A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections. (provider: SQL Network Interfaces, error: 26 - Error Locating Server/Instance Specified)
[REMOVED TEXT]
ClientConnectionId:00000000-0000-0000-0000-000000000000
Error Number:-1,State:0,Class:20
A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections. (provider: SQL Network Interfaces, error: 26 - Error Locating Server/Instance Specified)
After setting the Startup Project to Web and the Default Project in Package Manger Console I got it to work.

You may check service status of MS SQL Server 2014. In Windows 7 you can do that by:
Go to search and Type "SQL Server 2014 Configuration Manager
Then click on "SQL Server Service" on left menu
Check the instance of SQL Server service status if it is stopped or running
If it has stopped, please change the status to running and log in to SQL Server Management Studio 2014

Just restart SQL Server (MSSQLSERVER) service.

While the above solutions should work in 90% of the cases, but if you are still reading this answer!!! You are probably trying to connect to a different server than intended. It may be due to a configuration file pointing to a different SQL server than the actual server you think you are trying to connecting to.
Happened to me atleast.

When I experienced this error in Visual Studio,
“A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections. (provider: Named Pipes Provider, error: 40 - Could not open a connection to SQL Server)”
...it was during the execution of the following C# code, which was attempting to obtain my SQL Server data to display it in a grid. The break occurred exactly on the line that says connect.Open():
using (var connect = Connections.mySqlConnection)
{
const string query = "SELECT Name, Birthdate, Narrative FROM Friends";
using (var command = new SqlCommand(query, connect))
{
connect.Open();
using (var dr = command.ExecuteReader())
{
while (dr.Read())
{
// blah
}
}
}
}
It was inexplicable because the SQL query was very simple, I had the right connection string, and the database server was available. I decided to run the actual SQL query manually myself in SQL Management Studio and it ran just fine and yielded several records. But one thing stood out in the query results: there was some improperly encoded HTML text inside a varchar(max) type field within the Friends table (specifically, some encoded comment symbols of the sort <!-- lodged within the "Narrative" column's data). The suspect data row looked like this:
Name Birthdate Narrative
==== ========= ==============
Fred 21-Oct-79 <!--HTML Comment -->Once upon a time...
Notice the encoded HTML symbol "<", which stood for a "<" character. Somehow that made its way into the database and my C# code could not pick it up! It failed everytime right at the connect.Open() line! After I manually edited that one row of data in the database table Friends and put in the decoded "<" character instead, everything worked! Here's what that row should have looked like:
Name Birthdate Narrative
==== ========= ==============
Fred 21-Oct-79 <!--HTML Comment -->Once upon a time...
I edited the one bad row I had by using this simple UPDATE statement below. But if you had several offending rows of encoded HTML, you might need a more elaborate UPDATE statement that uses the REPLACE function:
UPDATE Friends SET Narrative = '<!--HTML Comment -->Once upon a time...' WHERE Narrative LIKE '&lt%'
So, the moral of the story is (at least in my case), sanitize your HTML content before storing it in the database and you won't get this cryptic SQL Server error in the first place! (Uh, properly sanitizing/decoding your HTML content is the subject of another discussion worthy of a separate StackOverflow search if you need more information!)

Why this error is so boresome and noisy, just because it can occur in varied situation.
I have done all approchs above here, and still being sucked. So make sure u have done the same as me before browsing downward.
Maybe I am not able to fix ur situation instantly, but I can point out a direction or thinking to u(The one who finally slide down here). I have started to ponder the error of my running program occurring after I made sure that the instance name is clearly right and set my database to allow remote control following the methods above. After then, I suspected something wrong happening in my code snippet of SQL connection.
Solution of my problem:
Check my sqlconnection function
Click to see its configuration
New a connection
Select ur server name
It works for me with pondering what exactly happen in the process of connection.Hope my thinking will lead u to kill ur error.

Try adding a , and a port number (as in ,1433) to the end of your connection string.

Summary
To fix this issue encountered while running local app vs remote database, use SQL Server Configuration Manager to add an alias for the remote database.
Details
I had run into this problem recently when transitioning from a Windows 7 to a Windows 10 laptop. I was running a local development and runtime environment accessing our Dev database on a remote server. We access the Dev database through a server alias setup through SQL Server Client Network Utility (cliconfg.exe). After confirming that the alias was correctly setup in both the 64 and 32 bit versions of the utility and that the database server was accessible from the new laptop via SSMS, I still got the error seen by the OP (not the OP's IP address, of course).
It was necessary to use SQL Server Configuration Manager to add an alias for the remote Dev database server. Fixed things right up.

I moved from a work laptop on Windows 7 to a work laptop on Windows 10.
I had been successfully using SSMS2016 on Windows 7.
The same issue was applied using SSMS2012 or SSMS2016.
My access to the 10 SQL servers using windows authentication was still the same. I could test this from another server.
However, 2 of the 10 servers would not connect from my laptop.
Both were ms SQL server 9 but I could connect to another SQL server 9 databases.
The solution was to add a firewall rule (using Windows Firewall with Advanced Security).
Create an Incoming rule for each SSMS
eg C:\Program Files (x86)\Microsoft SQL Server\130\Tools\Binn\ManagementStudio\Ssms.exe
I'm not a network expert so I've not included the details but hopefully, it will point you in the right direction.
Error msg (pre firewall rule)
"A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections. (provider: SQL Network Interfaces, error: 26 - Error Locating Server/Instance Specified) (.Net SqlClient Data Provider)"

If you're encountering this while debugging in Visual Studio, make sure that the project build path points to a local drive, or follow these steps to grant permissions to the network folder.

My issue started when I tried to change the server from IIS Express to Local IIS (while using LocalDB).
I was using LocalDB (for dev purposes), and when I went to revert from Local IIS to IIS Express, Visual Studio had switched my data source from Data Source=(LocalDb)\MSSQLLocalDB to Data Source=.\SQLEXPRESS
Incorrect connection string
<add name="DefaultConnection" connectionString="Data Source=.\SQLEXPRESS;AttachDbFilename=|DataDirectory|\SurveyTestsDB.mdf;Initial Catalog=SurveyTestsDB;Integrated Security=True;User Instance=True"
providerName="System.Data.SqlClient" />
Correct connection string
<add name="DefaultConnection" connectionString="Data Source=(LocalDb)\MSSQLLocalDB;AttachDbFilename=|DataDirectory|\SurveyTestsDB.mdf;Initial Catalog=SurveyTestsDB;Integrated Security=True" providerName="System.Data.SqlClient" />
Hope this helps someone out there.

Along with trying everything as suggested by Teo Chuen Wei Bryan, make sure you are also referring to the correct Server/Instance name in the connection string.
If you are using the short form of host name/Instance on the database server or in the web.config file, make sure you use the fully qualified domain name(FQDN)/Instance
Also, to test connectivity from a server where SQL server client is NOT present,
--> create a text file and change its file extension to .udl
--> Right click the file and you can see connection tab.
--> Input server name and log on information to test connection to the database server.
Hope this helps.

If you suddenly encounter this error say in a production environment and nothing has changed, try the following 4 items in the order below to see if it gets fixed.
restart the sql server service.
restart the service (say IIS) that is calling into sql server. (the problem is probably here if the time between the start of the service call to SQL server and the time you end up getting the response error is super short (about one or two second).
restart the server sql server is on.
restart the server the calling service is on.

Xml tag arrangement in Web.config is important
First
<configSections>
<!-- For more information on Entity Framework configuration, visit http://go.microsoft.com/fwlink/?LinkID=237468 -->
<section name="entityFramework" type="System.Data.Entity.Internal.ConfigFile.EntityFrameworkSection, EntityFramework, Version=6.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" requirePermission="false" />
</configSections>
After
<connectionStrings>
<add name="SqlConnectionString" connectionString="Data Source=.; Initial Catalog=TestDB; Trusted_Connection=True;" providerName="System.Data.SqlClient" />
</connectionStrings>

SQL SERVER 2008 R2 CAN'T CONNECT also cant repair it using setup [duplicate]

I get the following error when trying to connect to SQL Server:
A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections. (provider: Named Pipes Provider, error: 40 - Could not open a connection to SQL Server)
This error is thrown when I try to configure my database to gridview in Visual Studio 2010. I'm at a loss as to how to debug this error.
How would you debug this error? What steps should I take in order to determine what is really going on here, in addition to the one mentioned in the error message?

I found the following techniques helpful:
Make sure your database engine is configured to accept remote connections:
Start > All Programs > SQL Server 2005 > Configuration Tools > SQL Server Surface Area Configuration
Click on Surface Area Configuration for Services and Connections
Select the instance that is having a problem > Database Engine > Remote Connections
Enable local and remote connections
Restart instance
You may need to create an exception on the firewall for the SQL Server instance and port you are using:
Start > Run > Firewall.cpl
Click on exceptions tab
Add sqlservr.exe (typically located in C:\Program Files (x86)\Microsoft SQL Server\MSSQL.x\MSSQL\Bin, check your installs for the actual folder path) and port (default is 1433)
Check your connection string as well
From FIX : ERROR : Could not open a connection to SQL Server:
Check if your SQL server services is up and running properly:
Go to All Programs > Microsoft SQL Server 2008 > Configuration Tools > SQL Server Configuration Manager > SQL Server Services
Check to make sure SQL Server service status is Running.
In addition, ensure that your remote server is in the same network. Run sqlcmd -L to ascertain if your server is included in your network list.
Enable TCP/IP in SQL Server Configuration
When two or more SQL Servers are connected across network they do all communication using TCP/IP. The default port of SQL Server installation is 1433. This port can be changed through SQL Server Configuration Manager. TCP/IP should be enabled for SQL Server to be connected.
Go to All Programs >> Microsoft SQL Server 2008 >> Configuration Tools >> SQL Server Configuration Manager >> Select TCP/IP
Right Click on TCP/IP >> Click on Enable
You must restart SQL Server Services for all the changes to take effect. Right click and go to menu properties to select location where default port of SQL Server can be changed.

I got Solution for me :
Open "SQL Server Configuration Manager"
Now Click on "SQL Server Network Configuration" and Click on "Protocols for Name"
Right Click on "TCP/IP" (make sure it is Enabled) Click on Properties
Now Select "IP Addresses" Tab -and- Go to the last entry "IP All"
Enter "TCP Port" 1433.
Now Restart "SQL Server .Name." using "services.msc" (winKey + r)
It Will Work...

Adding my heavily upvoted comment as an answer with screenshots.
I spent a lot of time on this, finally what worked for me is:
1) Open Sql Server Configuration Manager --> SQL Server Network configuration --> Protocols for <(INSTANCE)> --> TCP/IP (double click on it).
2) Select --> IP Addresses(Tab).
3) Go to the last entry IP All and mention TCP Port 1433.
4) Press Win+R and enter services.msc.
5) Now restart SQL Server <(INSTANCE)>.
After this, the problem got resolved!

I am solving that problem by opening Services then start running Sql Server (Sqlexpress) service.

This Error mainly came when the SQL Service is stopped.You need to Restart the service.To go to this window you have to search the Services like this-
Then Search for SQLSERVER(MSSQLSERVER) and Restart the service.
Hope this will work.

In case you're using the Express Edition:
You need to add \SQLEXPRESS after your server name
e.g. MY-SERVER\SQLEXPRESS

Press window + R (Run window Open) and in run window type "services.msc" and new services open find SQL SERVER with instance name in my case it's SQL SERVER(SQLEXPRESS) then start this service and try again it works for me Hope Its Works for You also.

I had the same error when I wanted to run my WinForms project (that includes working with a SQL Server database and that worked perfectly on my PC) on another PC. The problem was in Windows Firewall on my PC. I solved this by adding two rules. This is the whole procedure how to allow SQL Server through Windows Firewall:
Open "Run" and enter services.msc
Find the service for SQL Server (instance name) and SQL Server Browser. One at a time, right click, select "Properties", copy the path to exe file
Then open firewall.cpl, click allow an application or add rule, add the previously copied path (there is a procedure you need to follow), check Domain and Private, uncheck Public.
This is the YouTube link where you can see this procedure: Allow SQL Server through Windows Firewall

I encountered the same problem
In my case, I solved the problem in this way
Step 1: From start menu went to SQL server configuration manager
Step 2: Enabled TCP/IP
Step 3: Double clicked TCP/IP and went to IP Address last entry IP ALL and entered TCP Port 1433 then applied
Step 4 : then pressed win+r and wrote services.msc opened the Services then scrolled down then right clicked on SQL Server (MSSQLSERVER) choose restart
That resolved my problem.
Even if doing all of the above steps do not solve the problem then simply restart the PC then hopefully it will work.

This solution resolves both issues Network Error & service behind SQL server
I answered a similar question here, you need to stat the other open Run type-> services.msc - under services -> sort by stopped you will see a bunch of stopped SQL services Right click and start
To begin - there are 4 issues that could be causing the common LocalDb SqlExpress Sql Server connectivity errors SQL Network Interfaces, error: 50 - Local Database Runtime error occurred, before you begin you need to rename the v11 or v12 to (localdb)\mssqllocaldb
Troubleshooting Steps
You do not have the services running run this cmd, net start MSSQLSERVER or net start MSSQL$ instancename
You do not have the firewall ports here
configured
Your install has and issue/corrupt (the steps below help give you a nice clean start)
You did not rename the V11 or 12 to mssqllocaldb/SqlServer
I found that the simplest is to do the below - I have attached the pics and steps for help.
Resolution Steps:
First verify which instance you have installed, you can do this by checking the registry and by running cmd
cmd> Sqllocaldb.exe i
cmd> Sqllocaldb.exe s "whicheverVersionYouWantFromListBefore"
if this step fails, you can delete with option d cmd> Sqllocaldb.exe d "someDb"
cmd> Sqllocaldb.exe c "createSomeNewDbIfyouWantDb"
cmd> Sqllocaldb.exe start "createSomeNewDbIfyouWantDb"
Restart MSSql Server or Simply restart your machine!
Hold/Press to open the CMD, window + R and Type "services.msc"
Now look for sql server services, open find SQL SERVER with instance name
then ReStart this service and try again

If none of the above solutions work (nothing worked for me) then just RESTART your computer and you will be able to connect to your sql server (localhost).

After doing everything mentioned here:
http://blog.sqlauthority.com/2009/05/21/sql-server-fix-error-provider-named-pipes-provider-error-40-could-not-open-a-connection-to-sql-server-microsoft-sql-server-error/
Still did not work for me.
Steps worked for me:
Start > Run > cmd > sqlcmd -L
It will prompt you the server name. Make sure this server name is same as the one you are trying to get connected to in CONNECT TO SERVER box of SQL management studio.
I made this silly mistake I keep using MSSQLSERVER rather using this server name.
Hope this helps for the people who make silly mistake like me.
Thanks.

I am using SQL Server 2016 and Window 10.
First thing is to allow remote connection to SQL Server.
What I did is to type sqlservermanager13.msc at start menu in order to open the SQL Server Configuration Manager. Make sure the TCP/IP status is enabled.
Check your TCP port number by double click TCP/IP protocol name. Usually it is 1433 by default.
The following procedures configure the Windows Firewall by using the Windows Firewall with Advanced Security Microsoft Management Console (MMC) snap-in. The Windows Firewall with Advanced Security only configures the current profile.
To open a port in the Windows firewall for TCP access
On the Start menu, click Run, type WF.msc, and then click OK.
In the Windows Firewall with Advanced Security, in the left pane, right-click Inbound Rules, and then click New Rule in the action pane.
In the Rule Type dialog box, select Port, and then click Next.
In the Protocol and Ports dialog box, select TCP. Select Specific local ports, and then type the port number of the instance of the
Database Engine, such as 1433 for the default instance. Click Next.
In the Action dialog box, select Allow the connection, and then click Next.
In the Profile dialog box, select any profiles that describe the computer connection environment when you want to connect to the
Database Engine, and then click Next.
In the Name dialog box, type a name and description for this rule, and then click Finish.
Another thing to configure.
To open access to SQL Server when using dynamic ports
On the Start menu, click Run, type WF.msc, and then click OK.
In the Windows Firewall with Advanced Security, in the left pane,
right-click Inbound Rules, and then click New Rule in the action
pane.
In the Rule Type dialog box, select Program, and then click Next.
In the Program dialog box, select This program path. Click Browse,
and navigate to the instance of SQL Server that you want to access
through the firewall, and then click Open. By default, SQL Server is
at C:\Program Files\Microsoft SQL
Server\MSSQL13.MSSQLSERVER\MSSQL\Binn\Sqlservr.exe. Click Next.
In the Action dialog box, select Allow the connection, and then
click Next.
In the Profile dialog box, select any profiles that describe the
computer connection environment when you want to connect to the
Database Engine, and then click Next.
In the Name dialog box, type a name and description for this rule,
and then click Finish.
Have a look at Microsoft doucmentation
Configure a Windows Firewall for Database Engine Access

You can test the following methods.
a
Check the connection string of the project.
b
Go to services and restart SQLServer Instance.
c
Open 'SQLServer Configuration Manager'
In the left panel select 'SQLServer Network Configuration' and expanding it
Select 'Protocols for MSSQLServer'
In the right panel dbl click on 'TCP/IP'
In the 'Protocol' tab set the 'Enabled' to 'Yes'
In the 'IP Addresses' tab scroll to down
In the 'IPAll' set 'TCP Port' to 1433
d
Open 'Firewall with advanced security'
In the right tab select 'Inbound Rules'
In the middle tab find the record that 'local Port' is 1433, If you can't found it try to created it with following levels
In the Start menu, click Run, type 'WF.msc', and then click OK
In the left panel click the 'Windows Firewall with Advanced Security'
In the right panel right-click 'Inbound Rules', and then click 'New Rule'
In the Rule Type dialog box, select 'Port', and then click Next
In the Protocol and Ports dialog box, select 'TCP', and select 'Specific Local Ports', and then type the port number 1433, Click Next
In the Action dialog box, select Allow the connection, and then click Next
In the 'Profile' dialog box, checking the Domain, Private and Public, Then click Next
In the 'Name' dialog box, type 'SQL 1433 Port' and for a description write description for own. Then click Finish
Then in the middle tab double click the found item(Instance) or created item of 'SQL 1433 Port' name by you.
Select 'Scope' tab in opened dialog box(SQL Server Properties)
In the Local PC Go to google.com in your Browser and search 'My IP'.
then copy of your 'IP'
Go to the remote server and in the 'SQL Server Properties' dialog box of 'Scope' tab, in the 'Remote IP Address' select the 'These IP Addresses' option and click 'Add' button
In the opened dialog box(IP Address) select 'This IP Address or Subnet' option and paste your 'IP', click OK button.

I have to run SQL Server Browser service into SQL Server Configuration Manager.
Installation can't discover newly created service without this.

I tried all the other answers on this question and some if not all probably played a part in getting this working for me, but I still couldn't connect to the DB remotely. I was using a SQL Server on an Azure VM.
I eventually remembered that the VM has endpoints that are controlled by the Azure account proxy, so I went on to the Azure Portal and added 1433 as an available endpoint and I could connect to my SQL instance.
Hope this helps someone who has tried all the other answers and is still having no luck!

I was experiencing the same problem and the problem was that I hade several projects in the solution (Weband Droid) and even though Default project was choosen in the Package Manager Console it used the connection string from the Droid project:
PM> update-database -Verbose
Using StartUp project 'Droid'. <-- DROID
Using NuGet project 'Web'. <-- WEB
Specify the '-Verbose' flag to view the SQL statements being applied to the target database.
<!-- BAD TARGET DATABASE -->
Target database is: 'DefaultConnection' (DataSource: .\SQLEXPRESS, Provider: System.Data.SqlClient, Origin: Convention).
System.Data.SqlClient.SqlException (0x80131904): A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections. (provider: SQL Network Interfaces, error: 26 - Error Locating Server/Instance Specified)
[REMOVED TEXT]
ClientConnectionId:00000000-0000-0000-0000-000000000000
Error Number:-1,State:0,Class:20
A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections. (provider: SQL Network Interfaces, error: 26 - Error Locating Server/Instance Specified)
After setting the Startup Project to Web and the Default Project in Package Manger Console I got it to work.

You may check service status of MS SQL Server 2014. In Windows 7 you can do that by:
Go to search and Type "SQL Server 2014 Configuration Manager
Then click on "SQL Server Service" on left menu
Check the instance of SQL Server service status if it is stopped or running
If it has stopped, please change the status to running and log in to SQL Server Management Studio 2014

Just restart SQL Server (MSSQLSERVER) service.

While the above solutions should work in 90% of the cases, but if you are still reading this answer!!! You are probably trying to connect to a different server than intended. It may be due to a configuration file pointing to a different SQL server than the actual server you think you are trying to connecting to.
Happened to me atleast.

When I experienced this error in Visual Studio,
“A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections. (provider: Named Pipes Provider, error: 40 - Could not open a connection to SQL Server)”
...it was during the execution of the following C# code, which was attempting to obtain my SQL Server data to display it in a grid. The break occurred exactly on the line that says connect.Open():
using (var connect = Connections.mySqlConnection)
{
const string query = "SELECT Name, Birthdate, Narrative FROM Friends";
using (var command = new SqlCommand(query, connect))
{
connect.Open();
using (var dr = command.ExecuteReader())
{
while (dr.Read())
{
// blah
}
}
}
}
It was inexplicable because the SQL query was very simple, I had the right connection string, and the database server was available. I decided to run the actual SQL query manually myself in SQL Management Studio and it ran just fine and yielded several records. But one thing stood out in the query results: there was some improperly encoded HTML text inside a varchar(max) type field within the Friends table (specifically, some encoded comment symbols of the sort <!-- lodged within the "Narrative" column's data). The suspect data row looked like this:
Name Birthdate Narrative
==== ========= ==============
Fred 21-Oct-79 <!--HTML Comment -->Once upon a time...
Notice the encoded HTML symbol "<", which stood for a "<" character. Somehow that made its way into the database and my C# code could not pick it up! It failed everytime right at the connect.Open() line! After I manually edited that one row of data in the database table Friends and put in the decoded "<" character instead, everything worked! Here's what that row should have looked like:
Name Birthdate Narrative
==== ========= ==============
Fred 21-Oct-79 <!--HTML Comment -->Once upon a time...
I edited the one bad row I had by using this simple UPDATE statement below. But if you had several offending rows of encoded HTML, you might need a more elaborate UPDATE statement that uses the REPLACE function:
UPDATE Friends SET Narrative = '<!--HTML Comment -->Once upon a time...' WHERE Narrative LIKE '&lt%'
So, the moral of the story is (at least in my case), sanitize your HTML content before storing it in the database and you won't get this cryptic SQL Server error in the first place! (Uh, properly sanitizing/decoding your HTML content is the subject of another discussion worthy of a separate StackOverflow search if you need more information!)

Why this error is so boresome and noisy, just because it can occur in varied situation.
I have done all approchs above here, and still being sucked. So make sure u have done the same as me before browsing downward.
Maybe I am not able to fix ur situation instantly, but I can point out a direction or thinking to u(The one who finally slide down here). I have started to ponder the error of my running program occurring after I made sure that the instance name is clearly right and set my database to allow remote control following the methods above. After then, I suspected something wrong happening in my code snippet of SQL connection.
Solution of my problem:
Check my sqlconnection function
Click to see its configuration
New a connection
Select ur server name
It works for me with pondering what exactly happen in the process of connection.Hope my thinking will lead u to kill ur error.

Try adding a , and a port number (as in ,1433) to the end of your connection string.

Summary
To fix this issue encountered while running local app vs remote database, use SQL Server Configuration Manager to add an alias for the remote database.
Details
I had run into this problem recently when transitioning from a Windows 7 to a Windows 10 laptop. I was running a local development and runtime environment accessing our Dev database on a remote server. We access the Dev database through a server alias setup through SQL Server Client Network Utility (cliconfg.exe). After confirming that the alias was correctly setup in both the 64 and 32 bit versions of the utility and that the database server was accessible from the new laptop via SSMS, I still got the error seen by the OP (not the OP's IP address, of course).
It was necessary to use SQL Server Configuration Manager to add an alias for the remote Dev database server. Fixed things right up.

I moved from a work laptop on Windows 7 to a work laptop on Windows 10.
I had been successfully using SSMS2016 on Windows 7.
The same issue was applied using SSMS2012 or SSMS2016.
My access to the 10 SQL servers using windows authentication was still the same. I could test this from another server.
However, 2 of the 10 servers would not connect from my laptop.
Both were ms SQL server 9 but I could connect to another SQL server 9 databases.
The solution was to add a firewall rule (using Windows Firewall with Advanced Security).
Create an Incoming rule for each SSMS
eg C:\Program Files (x86)\Microsoft SQL Server\130\Tools\Binn\ManagementStudio\Ssms.exe
I'm not a network expert so I've not included the details but hopefully, it will point you in the right direction.
Error msg (pre firewall rule)
"A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections. (provider: SQL Network Interfaces, error: 26 - Error Locating Server/Instance Specified) (.Net SqlClient Data Provider)"

If you're encountering this while debugging in Visual Studio, make sure that the project build path points to a local drive, or follow these steps to grant permissions to the network folder.

My issue started when I tried to change the server from IIS Express to Local IIS (while using LocalDB).
I was using LocalDB (for dev purposes), and when I went to revert from Local IIS to IIS Express, Visual Studio had switched my data source from Data Source=(LocalDb)\MSSQLLocalDB to Data Source=.\SQLEXPRESS
Incorrect connection string
<add name="DefaultConnection" connectionString="Data Source=.\SQLEXPRESS;AttachDbFilename=|DataDirectory|\SurveyTestsDB.mdf;Initial Catalog=SurveyTestsDB;Integrated Security=True;User Instance=True"
providerName="System.Data.SqlClient" />
Correct connection string
<add name="DefaultConnection" connectionString="Data Source=(LocalDb)\MSSQLLocalDB;AttachDbFilename=|DataDirectory|\SurveyTestsDB.mdf;Initial Catalog=SurveyTestsDB;Integrated Security=True" providerName="System.Data.SqlClient" />
Hope this helps someone out there.

Along with trying everything as suggested by Teo Chuen Wei Bryan, make sure you are also referring to the correct Server/Instance name in the connection string.
If you are using the short form of host name/Instance on the database server or in the web.config file, make sure you use the fully qualified domain name(FQDN)/Instance
Also, to test connectivity from a server where SQL server client is NOT present,
--> create a text file and change its file extension to .udl
--> Right click the file and you can see connection tab.
--> Input server name and log on information to test connection to the database server.
Hope this helps.

If you suddenly encounter this error say in a production environment and nothing has changed, try the following 4 items in the order below to see if it gets fixed.
restart the sql server service.
restart the service (say IIS) that is calling into sql server. (the problem is probably here if the time between the start of the service call to SQL server and the time you end up getting the response error is super short (about one or two second).
restart the server sql server is on.
restart the server the calling service is on.

Xml tag arrangement in Web.config is important
First
<configSections>
<!-- For more information on Entity Framework configuration, visit http://go.microsoft.com/fwlink/?LinkID=237468 -->
<section name="entityFramework" type="System.Data.Entity.Internal.ConfigFile.EntityFrameworkSection, EntityFramework, Version=6.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" requirePermission="false" />
</configSections>
After
<connectionStrings>
<add name="SqlConnectionString" connectionString="Data Source=.; Initial Catalog=TestDB; Trusted_Connection=True;" providerName="System.Data.SqlClient" />
</connectionStrings>

The same connection succeed and fails in different .net solutions

I have two .net solutions.
They both have this code:
var connectionString = "Server = ServerName; Database = DatabaseName; Trusted_Connection=True;"
var connection = new SqlConnection(connectionString);
connection.Open();
connection.Close();
And they both use complitely the same connectionString. And they connect to MSSQL Server;
And in the first solution connection.Open() succeeded and in the second one it failed.
Error message : Named Pipes Provider, error: 40 - Could not open a connection to SQL Server
And every time I create new solution it fails. So this connection only works in one specific solution. And also, my teammates tested this and didn't have such issues.
Can you help with hints why it can be so?

Check that your SQL server has the TCP/IP protocol enabled and if it is using dynamic ports, ensure that the SQL Browser service is running.
Make sure that any firewall software (windows firewall etc.) has an exception added for the particular exe (the sqlservr.exe file gets copied into the instance folder, which will be in C:\Program Files\Microsoft SQL Server\{instance name}\MSSQL\Binn, where instance name might look something like MSSQL11.SQLEXPRESS) or TCP port 1433 or UDP port 1434 (TCP 1433 for the SQL service itself assuming standard settings, UPD 1434 for the SQL Browser Service).
Check that the user account used to access the server (it should also have sufficient permissions and the login should be enabled), in this case it will be whatever windows account you are logged in as or whatever account your IIS application pool is running as because you are using Trusted_Connection=True.
Also check that you have an up-to-date SQL Client on your machine, you can get the latest version for windows here.

Open Sql Server Configuration Manager and make sure that the Named Pipes option is enabled for your Sql server instance.

Sorry for disturbing you. My problem was my inattentiveness. I created soluiton on another network machine.