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

If you com across this error which accessing your web application
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)

• Check your network connection 
• Check whether you have given proper SQL server instance name 
• Check your Sql server it might gone down or SQL service might be stopped.
  

Detailed article is here ...

http://www.sqlmusings.com/2009/03/11/resolving-a-network-related-or-instance-specific-error-occurred-while-establishing-a-connection-to-sql-server/