Fuse blown
Bad ground
Ignition relay
Starter solenoid
Ignition switch itself?
Could be one or more of the above. If you have an electrical schematic it should be helpful in tracing down the issue. That said, every time my car has had a starting issue it has been a corroded ground.....either at the solenoid itself or the block/frame, so start with checking that and the fuse perhaps.
I had a recent starting issue with my 853, which turned out to be a bad relay and incorrect ignition switch wiring (from previous owner)