In article ,
says...
For starters, do one of the several free online antivirus scans to see if
some 'virus' is preventing Defender from working properly.
I suppose you could also Google on how to remove and reinstall Defender.

I've scanned the machine with several free programs which
report nothing found.

Googling didn't turn up much (until your link below -
thanks!) that pertained to Win 10. I guess Defender was an
option in some earlier Windows versions and was
installable. That doesn't appear to be the case any more
(since 8?), becuase it's built-in(?).