I put my Rover into drive once in the CBD in Adelaide during peak hour when the lights went green. There was a bang, and the motor stalled. I tried to start it and it would not turn over. I got pushed off to the side of the road and called the RAA. I also checked the motor, to make sure it was not hot, and it was not, on the gauge, and by feel. RAA came and said that I probably had a transmission issue. I got it towed the next day to an auto trans specialist. They took out the transmission and found that I had a broken drive plate, that goes between the crank and the torque converter.
My P6 is a 2000, and the 4 cylinder model is a little prone to break drive plates. I have not heard of that problem occurring in the 3500's, Harvey P6 would know though.
I would hit the starter motor with a hammer and see if that makes it crank over. If that does not help then turn on you headlights, then try again and see if they dim considerably when trying to start it. If the headlight do dim considerably then this indicates that the starter motor is drawing current, but cannot turn.
What can also happen sometimes with pre-engaged starter motors is the solenoid will engage the pinion into the ring gear, but the contacts are burnt out which close the circuit onto the starter itself. It is sometimes possible to have an assistant to turn the key, and then using an old screwdriver, place it across the 2 contacts on the back. This may cause arcing and smoke. You need to have a steady hand to do this, and do not jump when you close the circuit. Sometimes this will work to close the circuit on a starter motor, which has a bad solenoid, and make it crank the motor. However I do not recommend doing this as it is dangerous. You may start the car in gear and get run over. You may well have access issues to the solenoid on the V8 model. I would really only attempt this if I was stuck in the middle of nowhere, and had to make it run to get it home. I have used this method with success on tractors, where you have better access to the starter motor, and you can easily jump out the way if you start it in gear.
James.