1 - The standard one: It's kind of hard for us to help based on the limited info you have provided. Basically you are telling us that you had a formula that worked, you changed it and now it doesn't work. You're not giving us very much to work with.
Perhaps if you posted the before and after, that might help.
2 - Have you tried the "Tools...Formula Auditing...Evaluate Formula" feature?
This will allow you to step through the formula and see the results of each part of the formula one by one. I have found this feature invaluable for both troubleshooting and for understanding formulae that I find in worksheets.