Actually I think it might be pertaining to this
http://support.apple.com/kb/TS3103Extremely frustrating and I think I'll pas in upgrading to Lion, judging by the negative comments I've seen from compatibility problems
Larry, I doubt that KB article pertains to your situation, as all you'd see with that situation is an inability to GET your mail - all your existing mail wouldn't have vanished.
I'm with Jon on this one - you may have directory errors and need to run Disk Utility (or preferably Disk Warrior) ASAP. In fact I wouldn't continue using your Mac until you've done those things, as there may be other errors that need to be fixed as well.
Do you have backups made prior to this? This situation is why I strongly recommend having redundant backups - preferably clones made with SuperDuper or Carbon Copy Cloner. One should be a daily backup and one a weekly one - so if you happen to not catch the errors/problems before the daily clone is made, you should have the weekly one to fall back on. (And don't do a daily clone on the day you do the weekly - again, so you don't end up with nothing but copies of the problems) Of course, best of all would be rotating weekly clones - so you'd have a 2 week old clone, a week old clone and a daily one. All of this can be done via automatic scheduling with SD or CCC. Drives are very cheap these days - losing your data isn't.
As for updating to Lion, there are always at least some compatibility issues with software with a major OS update. The only consideration is whether or not those problems apply to your situation. I'd also recommend updating either a clone or a machine that isn't crucial to your every day activities - that way it's easy to see if you do have any issues without impacting anything vital. I'll probably update this laptop, because I can always boot back to the clone of my Mac Pro (which is at home - I'm at the cottage for the summer) which I have with me if things aren't great, or if I need to get work done.
