Connecting sql server from NUnit test cases

I'm unable to connect to sql connection while running from NUnit test cases. My sql connection code sits in a different library.

Error - A network related or instance specific error occurred while establishing connection to SQL Server. The server was not found or was not accessible. Verify the instance name is correct and that SQL server is configured to allow remote connection.

I tried everything possible I found on the internet, couldn't resolve it. Please guide.

Thanks