The only 2 things that come into my mind are
1. Maybe there was a cross-link and your real wallet data is somewhere else on the HDD - for this you'll have to search on all sectors - used or not - for the magic numbers.
2. The file is encrypted and then you'll have to think harder on what you've done back then. If you would have used Rar or 7zip for this I would have seen their signature, but I don't know what you could have been using...