Reader, are you an idiot? Let me quantify that slightly. Are you the sort of person who gets up on a Saturday morning and puts a load of washing on without checking the pockets? And realises that your Fitbug is getting a wash it could have done without? I am.
I've done this before, and in the past, got away with replacing the battery. This time, though. pushing a new battery in made all the segments on the screen come on, and brought forth no response to the buttons.
So I took the whole thing apart and found it covered in white crud, presumably washing powder. Spraying the innards with WD40 drove that out, and re-assembling brought it back to life and saved me spending £30 on a new one.