OK ... I've dug my way out. Fortunately I'm rather anal about backups.
- I deleted the files I had just imported and closed LR (without doing another backup.)
- I erased my current LR Catalog file.
- I went to my backups and copied my last LR Catalog file back to the folder for my current file. (I make a backup anytime I make ANY changes to information in the files, so it was current as of just before I tried to import the new photos.)
- I spot checked several files from over the years and ALL appears to be in order. (Disaster averted ...)
NOW ... Anyone have any idea what the heck happened before I try to re-import my new photos?
David Marx answered this for me on Google+. (Thanks Dave!)
The only logical answer I can think of is that you accidentally selected every file in the catalog. I've done it myself in the past.
If it happens again the only change to your recovery procedure I'd suggest is to allow the backup to go ahead when closing and to rename your current catalog file rather than deleting it. I'd not delete even a majorly messed up catalog before I knew the Backups worked. Successive backups don't overwrite the previous one. I tend to backup daily and then weed them out to weekly, then monthly as time passes.