Can't see it from here, or your word-picture, but some obvious facts:
1) whatever shaft the sprocket is on is not free to turn.
2) some failure is causing that- like debris in gears, frozen motor or
Either it's absolutely frozen, and cannot be budged in either direction
(likely caused by frozen shaft bearing) or there is some angular play
at the sprocket. The latter would indicate problems further away, in
gear train or motor, or whatever else they put in there.
Really best you ask someone who can put an eyeball, and hand, on it.