all I can advise to try is:
- restore the file from your backup and see if you can open that version
- download the Excel viewer from Microsoft and see if that program will open the file
- copy the file to any other computer with Excel (by mail or diskette or USB-key) and see what happens if you try to open it there
- download OpenOffice and see if that is willing to open the file

If non of these work, consider the file lost, unless there's some clue you didn't tell.

Kees