Yesterday, I got the usual Windows message about needing to restart my Dell OptiPlex 3010 computer (running Windows 10 Pro); I accepted it, closed down all apps, and let it restart. After a while I noticed that it was hung on a black screen with only a circle of white dots revolving on the bottom of the screen. I left it overnight, but it was the same the next morning. I turned it off, left it for a minute or so, and turned it back on; it started to reboot, but when the blue window appeared in the middle of the screen, the circle of white dots appeared, with a brief message "Attempting to complete installation" (or some such). The white dots kept on circling for a few hours, and again I turned the computer off, waited a minute, then turned it back on, this time holding down the F8 key in hopes of getting into Safe Mode. That didn't happen, but this time when the circle of white dots appeared, under it was a persistent message "Recovering your previous Windows version". This time the reboot was successful, ending up with Windows the same as before, as far as I could tell.
So what was going on? Was there a massive SP-type upgrade that amounted to a new Windows version? Will the same thing happen on my other two computers, also running Windows 10?