Welcome to the forum.
I opened a session saved and files were missing from the session. Not only that, files from OTHER sessions were present in my selected session.
I have a hunch what may have happened. I left files open in the folder from another session instead of closing all. When I saved the next session, it may have saved those files as well. What is strange is that it only affected about half the files, not all the files of the mystery session. Solos are still present, but choruses from other vocalists are missing. Help!
Hmm.... I'd say that there's a moderately good chance that you've still got the original files somewhere, because in 2.0 anything recorded is written directly to file, regardless of whether it gets used finally in the session or not. And as others have found, you have to make quite an effort to lose stuff, even when you want to; if you do drop-ins, there's no easy way of deleting your unwanted takes, for instance - they are all still present in the recorded folder, taking up space, even though they don't show in the session at all.
It does sound as though you may have a corrupted session file, though. And if this is the case, your only recourse is to locate the original files, and re-create the session. But as I said, I think that if you look in the folder, all the files you recorded are very likely still to be there. Okay, that would be a bit of a PITA, but it's a lot better than breaking some rather worse news to the artistes, so it's well worth doing a thorough investigation first.
Rule one of session recording in these circumstances - make a backup folder of the session at the end of each one, preferrably on a different drive (or onto DVDs). I think that most artistes would forgive you for having a corrupted session file, as long as the original material they recorded is intact.