So - now that I've calmed down and accepted the loss, and have thought about it more:
What happened was that it went to a light blue screen (NOT the BSOD screen) and said in white type "Updating now" or something very similar.
It looks similar to that when Windows is updating.
I did a System Restore after that, and it took many hours - which makes me think Windows had downloaded files without notifying me or asking permission, and for some reason, after Avast updated 7 Zip, Windows decided it was time to take over the computer and install/update Windows files.
Windows forced updates the culprit? I've had them wipe out files in the past. But this one happened immediately after Avast had 7 Zip update.
So if my headline is in error, my apologies to Avast.