Hi Heather and welcome to the forum,

"Won't work" has a lot of possible parts to it. Here are some things to try or look for:
1. Is Calculate set to automatic or manual?
2. Are there named ranges in the formula that doesn't work?
3. What is the formula, exactly, that doesn't work?

Posting a sample workbook that shows the problem is most likely the only way we can really diagnose the problem. You can submit a sample by clicking on "Go Advanced" and then on the Paper Clip Icon (if it shows) above the advanced message area.