Last night I jiggled, wiggled, and pounded on it and finally got it to turn. I think the last post is correct, it's worn to the point where it will only work the latch when it's turned all the way ccw (past the horizontal; it won't open with the lock turned to the "unlock" position, only when turned about 45 degrees past that). Still don't know why the key wouldn't turn, but since it's effectively "locked" all the time I'm not going to turn it vertical again for awhile. I'll try removing the latch and seeing if I can figure out where the wear is.