Excel Issue

User experience problem with excel. They will be working on the file and all of sudden the font will change to very extra large and the number will take most of the screen, Then error message comes with one option which is to close, and some times they can?t even exit. After that, user looses all the work and change they made to the file. This issue happens 6 months ago and stays for 2 weeks with so many users. Just 3 weeks ago users reported same issue. Not sure what cause this, we use office 2002 and all the files on the network. One user reported this issue with a file he created on his local system. It seems to me this issue happens if the file contains a lot of format not 100% sure. I do not know where to look or where to start, is it network issue or Excel?? Please help.