My problem began a couple of days ago. There was no indication what caused it, and no sign of trouble proceeding it.
I attempted to reinstall Windows 10 (keeping existing files) over my current Windows 10. That didn't do anything. Then I tried your suggestion. Although the sfc /scannow said there were corrupt files that it fixed, the problem remains as it was.
BTW I'm connected via ethernet. Ever since I upgraded from Win 7 I had intermittent trouble with my Netgear a6200 keeping a connection. That may or may not have anything to do with this, but it still sucks.