Missing Method Exception was unhandled by user code?

I think the error is telling you everything, you're trying to call a method in a class that doesn't exist, try to recompile your whole solution and not just the project.
If you're using VSS or something similar try to update your files to the latest version and then recompile, if that doesn't work, well, debug your code line by line where it happens.
 
Giving no code is like trying to drive a car while you're blind