There are multiple motors in the lower track, likely one has stripped its clutch. GM sells the individual motors on some models and only the track assembly on others. You would need to check with a dealership parts department to see which. You could also check a salvage yard, locally or on line.
If you just want it back in position, you can normally remove the motor, move the track and reassemble. It wont move, but at least it will be in position.