Howdy, and welcome ot the board. I use a Mac at home. Couple of questions:
What system do you have?
What version of XL?
Have you updated with all the latest patches from MS for your version of XL?
I use XL 2004 on OS 10.3.9 and have never had a problem. But it sounds like your file might corrupted. If so, here are a couple of links that might help:
J. McGimpsey
Jim Gordon
Both are Microsoft MVPs for the Mac Office.