(for CT412):
This has happened to me on occassion, and I finally tracked it down to an otherwise pretty handy utility called Registry First Aid, which runs through your entire system looking for broken, outdated or nonfunctional registry entries and then gives you the option to delete, edit etc them. Although in general such house cleaning can greatly improve an operating systems speed and functionaility, caveats are of course in order when messing with the registry; and the only real problem I've noticed is with Sidekick. For some reason, ---and can't remember the precise nature of the problem, but--- the program would find several Sidekick entries as in error, and not only that, but would mark them as safe to fix (it does try to warn you of potentially unsafe changes, but not with Sidekick), but if you accept the default "fix" the next time you open Sidekick you get blank templates for Calendar, Notes etc, and all your data has disappeared.
But not really: as mentioned by Frank10, if you can locate the files on your own hard drive (or on your back up) and just open, then save, them, you should be fine. And if this IS the problem, you should be able to open the WORKING files, not just the BACK-UP (in case your back up didn't include recent data, try to find the working ones first so you don't lose any unbacked up data).
Hope that helps.