Instance Failure MSSQL Server in C# ASP.NET - DotNetNuke

Check your web.config file for the following:

connectionString="Data Source=.\\SQLSERVER;Initial Catalog=Northwind_db_;Integrated Security=True"

The problem is "Data Source=.\\SQLSERVER;" - It's the double \\. That's an escape sequence in C#. Switch the connection string to .\SQLSERVER.

If this is not the case please check to make sure that your SQL Server Service is running.