Disconnect the belt and operate the treadle, it should move smoothly once you get it going and it should move for a little bit if you lift your foot while operating. While the belt is off turn the machine by hand, it also should turn smoothly.
The belt is slipping: 1. on the treadle pulley, 2. on the machine handwheel, 3. on both.
If it worked fine before laying it on its back and doesn't now, this is where I would start to try to isolate the problem.