"IF" the switched hot to the solenoid is dead (just disconnect it for trouble-shooting this...), and its cranking, the contact plate inside the back of the solenoid is welded to the main power and motor jumper lugs. When the solenoid is sent power it acts like a high current relay for the starter motor. The starter solenoid is engauged, energizes its electro-mag, pulls back on a fork that pushes the bendix assy forward, shoving the pinion gear in mesh with the flywheel. At the same time, a contact plate inside the solenoid closes the high current starter motor circuit, by connecting the main power cable, and the jumper cable going from the solenoid to the starter motor (I think these are usualy 6A or 7A cables?).
Anyways, smack the solenoid with a hammer toward the rear of the housing close to the big power studs (location of the contact disc), should/might knock the plate loose. Long term fix, remove the solenoid, dissasemble, and flip the plate around, using the side that has never made contact with the main power lugs, good as new.....