Engine grounding caused this problem on mine. The chassis-engine strap was loose. You could try connecting a jumper cable from the engine to the battery and see if that will get things cranking.

A good rap to the starter with a rubber mallet, while someone turns the key, may free the bendix if that is hanging up.

Also, a broken terminal connection at the starter could be possible.

__________________
Sam

84 300SD 350K+ miles ( Blue Belle )