1st recommendation: do a redundant backup now, i.e., 2 copies in removable media
2nd one: copy a few files and test them in other computers and/or upload them here
3nd one: uninstall and reinstall Excel
4th: ran an online antivirus, then update your AV definitions
5th: same for malware detectors (i.e., Spy Bot Search & Destroy)
6th: general backup
7th: format disks and reinstall Windows and software

If in any step you find a solution then you're safe, with less or more work but safe. If still in the same conditions, well, I started saying it was very improbable, but not impossible.
As additional resource where you may find more ways for corrupt excel documents https://social.technet.microsoft.com...at?forum=excel