Have you tried looking under Lightroom >>File>>Open recent ?
If you remember the Lightroom catalog name try searching it on your computer.
Yes, Open Recent shows only the default catalog name, which has now been replaced with an empty one. I've searched the entire hard drive for other .lrcat files, and the only ones are my backups. I've restored my November backup and am now in the process of adding in photos from the last three months. This is a hard reminder to back up more often. And I guess rename my catalog something other than the default!
Are you sure your catalog was not stored on External drive?
Try locating your catalog Recover catalog and images after resetting preferences in Lightroom
Could you find you catalog in Pictures folder.
Why don't you turn on the Lightroom feature to make catalog backups on a regular basis (for example, every week)?
I've looked and looked. No external drive attached. No other lrcat file. But I've now restored the backup, adding in newer photos, and BACKED UP!
I have no answer as to why this would happen in Lightroom -- an automatic update (I assume) overwriting the existing LightRoom.lrcat file with a blank one. But at least I've recovered my work, and I'm happy to see that the ratings and captions were stored in the photo files, so they are there.
When you say "I've looked and looked", I kind of get the impression this is a manual/visual search where you look in a bunch of folders. You really ought to use your operating system's search features to search for .LRCAT files. Is that what you did? Or not?
No, I did a computer search of both my drives and only found the empty catalog file.