Calvin uses Adobe Illustrator CS5, and his files won't open anymore. Leo says it sounds like the files have been corrupt or damaged. It may also be that CS5 can't read older CS2 formats without converting them. Backups don't work either. Leo says it sounds like the files got corrupted, and then synced to the backups, overwriting the healthy files.
Adobe has a tech note "Troubleshooting Damaged Illustrator Files". It's possible that reinstalling the software will help.