It’s a Pi Zero 2 with an HDMI screen, so either… the Pi has gone bad, or the screen has. Oh, well, I foresee some debugging in my future.
[Update some hours later: I pulled the plug on it a few times, and now it works perfectly again. *sigh* That’s really the Raspberry Pi Experience — they mostly work until they don’t, and you never find out what the real problem is. Power? Cables? Heat? Coldth? Nobody knows.]