1. Some oddity caused during installation of ActiveStinc (and ActiveSync Toggle to address the fact that, no, I don't need it using up computing power 99.9% of the time) to enable direct connection of our Wings and the desktop (until now, we've only used the laptop for that purpose).
2. A fan/heating issue.
3. Power failure caused by UPS failure or feline intervention (the big red power button is on top of the UPS unit).
It didn't give any of the "Windows did not shut down normally" messages that would result from a crash or power cut, which leads me to suspect #1. However, it seems to have stayed on all day after my reboot this morning.
As for why I'm here posting this instead of off with Three Left Feet -- apparently the northbound Yellow Line trains weren't running even to the usual lackadasical schedule, and the information board didn't indicate any coming for at least the next 15 minutes (i.e. about the time my last good "bail and make my way home" window closed).