My 1988 560 did this same thing on Sunday.
No start, dry plugs, jumped the fuel pump, pumps were running and depressed the airflow plate- Nothin. No singing injectors, no nothing.

I had to go get all my tools and fuel gauges, hooked up the fuel gauges, jumped the pumps again and found the fuel pressure was correct. At that point I deflected the airflow plate and Voila. Singing injectors.

Disconnected everything, put the air cleaner back on, closed the hood, jumped in and it started right up. (was this some kind of automotive test of will??).

Why the injectors didn't flow with the pumps on and airflow plate depressed is a good question.

Can an EHA "stick" closed and bypass all the fuel pressure causing our no-starts?