ctopher,
The only "backup" consisted of an older version saved under a different file name, about a week ago (I lost about a week's worth of revisions to it).
I read in another thread that it would have been wise to have the "save backup copy" option turned on. Merely saving as a different file name is just as good.
Some possibilities that may have caused this: I was in the process of copy/pasting a worksheet from an Excel 2003 file to an Excel 1997 file. The "ASAP Utilities" add-in was running.
For some odd reason, this morning, none of my Excel files would open properly. I then tried uninstalling ASAP Utilities, and problem was gone (what the heck happened)