If anybody has any ideas on how to fix this problem could they please let me know.

Well Johnathan,
Lots of monks have already chimed in on this with the right answer, but let me add some thoughts anyway.

Always, always, always, have some validation methods available to you. In this case I would have had some XML that I know works to validate that it still works.

If you have a test suite available to you that you've developed then you can be more confident in your code.

You didn't post any code so I can't tell if you did anything to catch errors to avoid the Error 500 Internal Server Error bomb. Of course the crystal ball doesn't always tell you ahead of time all the wonderful ways users can break your code.