I didn't install a new OS, this was just one of M$'s updates to W10. On reflection, I installed an update to BluOS a few days ago and maybe that's the culprit?
Depending on the exact versions involved, a Win10 update effectively can be a new OS install. Might be worth checking for a
C:\Windows.old folder, though Documents should
not be moved there. "Should."
I'm no fan of how MS has handled updates & QA over the last few years, but they are fighting statistics here. With hundreds of millions of installs, even 99.99% reliability causes trouble for many thousands of users.
Finally, let's recall the 2 types of computer users:
1) Those who make regular backups.
2) Those who have not yet lost important data.