Why would excel 2007 automaticly change formats?

Upon opening a workbook this morning all the blank cells in the 16 sheets
had changed from general format to date format, and 99.9% of cells with data
or formulas were also changed to the date format.
I opened another workbook and it was find, I closed it and reopened it and
the same thing occurred, most cells had been changed to the date format.
The first wb containts payroll data, with mostly sumifs functions and was
working fine yesturday. The second wb contains data using vlookup and if
functions.

Thank you
Deb