It's down now. I'm not sure what the problem is. Disk error, perhaps. This may be serious. I'm connected but there's not much I can do. I'll let you know.
That was really odd. The disk was in a peculiar state and very few commands functioned. Notably, I couldn't reboot it using any of the usual commands. (It's in a locked machine room at a colocation facility three miles from here and it's after 11:00 at night so pushing the button was not an option.) I did manage to force it into a reboot state somewhat more directly, and it came back promptly with everything seemingly fine. I'm doing an unscheduled backup now since today's didn't have a chance to occur at its normal time.