Could be the motor brushes. Unscrew the little cap, but hold it down so the spring doesn't fly out. There should be a carbon brush on the end of the spring. Don't want those to wear all the way off, or you'll damage the armature.
Try turning the motor pulley by hand. It would be easier to test if you can disengage the belt. Is the motor pulley turning freely? If not, there could be some thread tangled around the pulley or the shaft, or the bearings need some grease.
If it was at the shop, they would try a different foot control, to make sure that's not the problem.
Good Luck!