Windows updated itself and went in a boot-crash loop. It did detect it was hosed, but failed to roll back, go to a restore point, or even reset properly. Three hours later, on a hunch after some Googling, I unplugged the USB cable to my monitor and everything is "fine." Obviously.
Pretend I rant here about how Microsoft and United Airlines are trying to outdo each other in antagonizing their customers. I'm too tired to type it right now.