Number Format Problem

Hi everyone,
For some unknown reason, my computer has just save two large workbooks with all general format cells now showing as dates. This could mean hours of work to manually re-format, and I have no idea how or why this has happened so it could happen again I am guessing. Can anybody explain why this has happened and if there is a way I can rescue my workbooks? As far as I am aware, I havent done anything to change defaults or anything. Apparantly this happened to a workmate of mine and it seems to happen from time to time with excel 2007. Can anyone help?
Cheers in advance,
John.

can you reformat every cell to general, then reformat essential ones ?