Thanks, I'll have to remember that next time, though I'm still a near-total newbie at using command lines (and Linux in general, for that matter).
However, at the time this happened, it wasn't nearly as bad as you might think, because 1) I was running a fairly new installation of Raspbian on my Raspberry Pi; and 2) experience has taught me not to put any documents on the same disk as the OS, so what little that was worth saving was already on a separate USB drive.