In my Roomba 620 one of the wheels stopped turning because it came off it's shaft. You can diagnose by turning the wheel by hand - it should turn loosely without much resistance, compared to the other wheel. I fixed this by cleaning the shaft and the socket and then attaching the wheel back. I had to press very hard to get the wheel far enough on the shaft. It's really weird that the wheel came off like that considering how tightly it fits. Maybe it was poorly attached initially.