I had an identical situation a month ago. However in my case I had been replacing parts (Fuel dizzy, EHA, injectors, etc). Drove it for a few days and then one day just wouldn't start.

Its pretty much gotta be 1 of 2 things: Fuel or Ignition.
Assuming your airflow plate is moving freely?

I confirmed the fuel pumps were running and even jumped the relay to manually keep the pumps on with no change in starting.

I pulled a spark plug and it was BONE dry.
This indicated to me there was probably spark, but no fuel going to the cylinders.

I Jumped the fuel pump relay to engergize the pumps and depressed the airflow plate and nothing happened. The tell-tale "singing" of the injectors wasn't there. They make a distinct sound with the pumps on and the more you deflect the airflow plate (you will be flooding the cylinders).

I opened the fitting on the side of the fuel dizzy for testing fuel pressure with a 12mm socket, relieved all the pressure, closed it and tried again. Fired right up.. Never done it again.

Maybe the EHA was stuck? or the DPR?
Got me, never done it again, but I re-adjusted fuel pressure differntial after that.

Pull a plug first: Wet = ignition or flooding. Dry = fuel flow.
Wish I could be there to help.

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Thomas Johnson
Phoenix, AZ USA
1988 560SEC