Sounds like the last time it was being used a piece of old belt tore off and wedged itself in the sander locking the belt wheel from turning, unplug and try to clear it out,you may have to take off a shield to gain access.
If its not a stoppage of the belt wheel, it may need new brushes, that would be the next thing to test after it has been cleared for any obstruction, generally brushes cause a nasty smell when they are burnt out, unplug the tool and open up the slotted ports that contain the brushes, one on each side of the motor, replace if they are worn too far down.

Likely the gear belt that drives the drive rollaer has failed. Either it is totally gone or the teeth in a spot have been destroyed and the motor is just spinning the sprocket in the belt. The belt is about 1/2 inch wide and is covered by a belt guard.