Library didn't come back on it's own. But that's because I had deleted the Media Center 21 directory (from ~/Library/App...../). I had tried restoring that directory (with Time Machine), but MC still wouldn't launch. All of this prior to rebooting.
The reason I thought the library was corrupted was because MC made a new directory called "Read Only Library" in the Media Center 21 directory. I'm not sure what happened.
After I got it all working again, I restored my Library from a backup made about 24 hours ago. I found that it was missing a few things: It was missing one of my zones, that I had to rebuild by hand. But I *also* had an accident yesterday where pressing the delete key deleted that zone. So that's probably why the zone was gone. Actually, now that I think of it, it was also missing a 3rd zone I use that I had to rebuild. Strangely, I also found that it reverted my "Customize Display" settings for Standard View. I had to cut and paste those from notes I had made.
Otherwise, I don't think I lost anything, other than some library stuff that I did between library backup and library disaster. When I'm more awake, I'm probably going to install V48 again and see what happens.
Brian.