Win32Exception (0x80004005): The network path was not found

while running my asp.net application package on test server I am getting "Win32Exception (0x80004005): The network path was not found" error and then saying: "[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: Named Pipes Provider, error: 40 - Could not open a connection to SQL Server)]"

while publishing in publish web settings I checked "Use connection string at runtime " and " update database configure database updates" and in preview also my database is coming. But on deploying it on server I am getting error.

I already tried to change the connection string in web.config file but it didn't work.

please help.