I'm facing kind of the same problem, except that my playlists (smart lists) are still there, but they've lost all their references to the files (FLAC/mp3) themselves, so of course they are now unusable. I can't even imagine the numbers of hours wasted.
I know I can "just" restore an old backup, but really, this shouldn't be the answer. At least you could try to explain us why this is happening. Is it a bug or something related to a feature change? Or maybe a result of a crash?
I've been with you since MC12 (at least), and clearly MC18 is the least stable release until now. I can live with the frequent crashes and a Television feature that seems to live an unpredictable life on it's own, but when my library gets messed up it's really bad.
So sorry to rant. Really. But like CharlieM I'm "very upset".