I would suspect that the bendix drive on the starter may have jammed. Just remove the starter, inspect the mechanism which engages and disengages the starter and if no
parts are broken, just give it a clean and a spray of lubricant and it should be OK.
If it has not been inspeted for a long time and is old, it would probably pay to check the
brushes and commutator whilst you are at it.The commutator shoud be bright and smooth, and the
brushes should be within the makers length spec.
Regards,
Richard.