It restarted successfully and is running its backup programs a little early, but (1) that was way more exciting than I needed tonight and (2) I'm guessing that this is the HDD error that came up about two months ago and that I decided to ignore until I couldn't any more.
Honestly I'm half-tempted to just bring the netbook—which lives in the Pip's room for me to use at naptime, bedtime, and in the middle of the night—downstairs, hook it up to my monitor and external hard drive, and just prop the tablet up in his room. But I should really at least open the case and see if any fans are obviously broken or dust bunnies are breeding or whatever, before deciding whether to replace the HDD. Again. Whee.