If you are getting this error from the server right after installing SP1, you can open the event log and check for the crash report that has been created with the error details.

If the error is similar to

Exception: TypeInitializationException
Message: The type initializer for 'Microsoft.TeamFoundation.Server.IntegrationResourceComponent' threw an exception.
Stack Trace:
   at Microsoft.TeamFoundation.Server.IntegrationResourceComponent.RegisterExceptions()
   at Microsoft.TeamFoundation.Server.Global.Initialize()
   at Microsoft.TeamFoundation.Server.TeamFoundationApplication.Init()
Inner Exception Details
Exception: ReflectionTypeLoadException
Message: Unable to load one or more of the requested types. Retrieve the LoaderExceptions property for more information.
Stack Trace:
   at System.Reflection.Module._GetTypesInternal(StackCrawlMark& stackMark)
   at System.Reflection.Assembly.GetTypes()
   at Microsoft.TeamFoundation.Server.SqlResourceComponent.RegisterExceptions(Assembly assembly)
   at Microsoft.TeamFoundation.Server.IntegrationResourceComponent.RegisterExceptions()
   at Microsoft.TeamFoundation.Server.IntegrationResourceComponent..cctor()

This sometimes happens when you have BOTH client and server on the machine, but you installed SP for the client only. The best way to fix this is to install TFS SP1 on the server machine as well..