Got to be some brushes and an armature if you can get in to them. See if you can clean up that armature (to shinny copper) and see if there is anything left of the brushes too.
Can probably replace the the brushes if you ultimately have to.
Most basic make sure the brass bushings (bearings) on the armature are not stuck keeping it from spinning. They can be cleaned and re-greased too.
Also check to see the plastic gears are meshing properly, they could be stripped and jambed.
Kind of like cleaning up and old GM starter. Used to do it all the time:
Clean and polish armature Contacts.
Replace Brushes
Lube or replace brass bushing bearings.
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