Sounds indeed like you upgraded an old catalog. Maybe not from a backup, because backups are zipped so you can't select a backup copy by mistake, but perhaps the old catalog from Lightroom Classic 10 that was still in your catalog folder.
This appears to happen on many if not all upgrades, not the updates, but the upgrades. Users find their "upgraded" catalog is missing images, missing edits, missing folders, or even completely empty (as if new). In the last few days (day?) you will find several similar postings.
I suspect what happens if your preference file is corrupt in just the wrong way, LrC has issues upon upgrade opening the correct catalog, or even any catalog.
I assume you started LrC by clicking on the start icon, the shortcut, and LrC started, then tried to open the most previous catalog (default setting), and it went downhill from their. This would be the norm (start method, that is)
If you still have v12 installed, instead of launching LrC via the shortcut, use your OS file manager and bring up the correct catalog, the v11 catalog, double click on it, see what occurs.
You might need to reset/refresh that preference file.