Look for any files in the following:
C:\Documents and Settings\username\Application Data\Microsoft\Excel\
In my case, I found an errant file here:
C:\Documents and Settings\username\Application Data\Microsoft\Excel~ar3451.xar
For an Excel 2003 file on our office LAN the computer blue screened and all changes were lost to the document. I found the file shown in the above string and right-clicked, Open With Excel and voilà, there was the file with all of my changes.
Immediately saved it with new filename and back to work.
