Are you saying that you turn the key and the starter does not engage? If so, that could be the NSS (neutral safety switch) gone bad or it could be worn shifter bushings (ie the vehicle does not know exactly what gear it is in). If you can find a sweet spot with the shifter where the starter fires, then it is likely shifter bushings.

If you have to jump it with two screwdrivers from the jumpbox under the hood, it may be the ignition switch.

Just a thought from my experience with my '81 300SD over twenty years of ownership.

Good luck. Others will chime in with better info.