After work I walked the bike over to the shop. It turns out the shifter cable was fraying. The guys said it can happen very suddenly. That seems to be true, because I had the bike tuned up just two weeks ago.
I also did something funny to my right knee; pulled up a bit too hard on the pedal, I think, during one of my build-ups to 20 mph. It seemed to ease out after several more miles of riding at a more moderate pace, but when I rode the hybrid home from work I noticed it again. I'll watch it for a few days, but I don't think it will need a doctor -- thank goodness!!