This didn’t get a lot of attention but I’ll add a quick follow up. Sort of a 'doh! moment.
My problem was my monitor. It quit. It didn’t occur to me that it would quit by failing intermittently. I figured out a keyboard shortcut sequence that I could use (without being able to see the desktop) to log in (if the lock screen was on), make sure that the focus was on the desktop (so no open window would take the keyboard shortcut I was using) and lock the screen again. This had the effect of goosing the display and it would return to normal for a while. I could log on and it might not happen again for days or a week. I figured something in the OS had gone buggy.
Nope. It started happening more often then then, all at once, the display would blank after a couple of seconds over and over no matter what I did. I switched monitors and 'doh! It was the monitor. It took me a long time to ask - could it be the monitor? It’s always in the last place you look.