LINQ in ASP.NET — throws exception when deployed to the server

I am connecting to a SQL Server database via LINQ in my ASP.NET application (returning results to a web service). The problem is that it only returns results when I'm running it in debug/test mode - When I publish the service I get an error telling me "Login failed for user 'NT AUTHORITY\NETWORK SERVICE' "I am connecting to a SQL Server database via LI