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.