Error: A network-related or instance-specific error occurred while establishing a connection to SQL Server.

Error: 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)


This is the common error for sql server while establish connection from visual studio to sql server.


If you encounter this error, while establishing connection from visual studio to to sql server, than first things to check in web.config file that have you set sql server instance name properly and correct?


You can encounter above error if you have 


Change your PC name or sql instance name
or you have installed new isntance of sql server.


EG. Currently your PC name is 'PCName', and sql instance name is 'SQLServer'
Now you are changing your computer name from 'PCName' to 'MyPC' or sql server instance name from 'SQLServer' to 'MySQLServer' then you must change connection string 
in your web.config file  like below sample connection string


Old Connection String:
Data Source=PCName\SQLServer;Initial Catalog=SampleDBName;Integrated Security=True


New Connection String:
Data Source=MyPC\MySQLServer;Initial Catalog=SampleDBName;Integrated Security=True


Solution:
First check your sql server instance name after opening sql server management studio, then check connection string in your vs.net solutions,
then compare both instance name if both instance name are not same then please make both name same and save changes you made in web.config file and run the project