I am soooooooooooooooo stoooooopid.
I thought I'd have one last try before calling an electrician, because I'm a poor miser and hate wasting money on things I could fix myself.
So I go down to look at the fuse box. Now note that the fuse box is not labelled, so I was merely guessing... Last night when I tested it, I pulled the first 5 amp fuse I could see, and the lights did not go off downstairs, so I assumed (stupidly) that it was the upstairs lights fuse (all lights are on 5amp circuits here). But of course there are actually three 5 amp fuses there. I hadn't seen the third one, or just blanked it out, or something. So just now I of course discovered the blown fuse, re-wired it (shoulda just stuck a damn paperclip in it!), and now we have light again.
I'm dreading telling my wife. I feel so dumb.