Lo and behold, the spring that opens the garage door had gone ka-zoing! Last year, the loop on the stationary end broke. Then tonight the loop on the other end broke.
Take a look at the carnage. The force of the breakage must have pulled the nails out of the ceiling and bent the stabilizer board backwards. Or perhaps the detached board caused the breakage. The good news is that the spring appears to have hit the dirt sifter instead of the car. Whew!
Anyway, I had to remove the flailing board, pry out the nails, and then reinstall the board with 3" screws. I think I'll examine this whole setup again in the spring. Oh, yeah, did I mention it's the coldest night so far this winter?? Brr! At least most of the work could be done with the garage door closed.