Assuming yours is a relatively modern powerplant, my guess is a faulty diesel cutoff solenoid, or its connections.
Most modern diesel engines cutoff systems rely on a normally open solenoid to actuate an auxiliary valve, usually located at an acessible point of the injection pump body, which breaks diesel flow when actuated, thus bringing the engine to a halt.
Out of curiosity, this is why modern diesel engined cars with a completely dead battery often fail to start by pushing them: The solenoid needs at least some energy from the battery to open the valve and allow diesel through.
Older engines used to have a mechanically actuated decompressor which actuated by means of a steel cable or rigid rod connection and (almost) never failed.
Anyway, if you look carefully you should be able to find the place where a wire connects to the injection pump. That's where your solenoid is. Simply turn your ignition switch on (without cranking the engine) and search for an audible "click". The same should be heard when switching back to off.
If no sound is heard you probably have a faulty solenoid giving you a headache. If you're familiar with simple troubleshooting procedures you should then jumpwire the connections to try to understand if the problem is from the solenoid itself or its connections, if you don't you should in your own interest get the engine serviced by an electrician...
We have a saying here at our sailing club that goes: The worse part of any sailing boat is its engine
Hope to have been of any help
Regards from Portugal
PS:Curiosity: How did you manage to kill the engine when the cutoff first failed?