I guessed the belt had broken. But actually, the pulley was loose... the metal fatigued and scored. It finally got so loose, the belt jumped off. The repair was very simple. One nut held the old pulley. Tapped the new one on and tightened the bolt. Put on the new belt - good as new! Pro tip: Hold the belt on the pulley with a zip tie. WAY easier to get it on! (Just don't forget to cut off the zip tie when you're done!)