Diesel fuel problems

What could be the diesel fuel problem beyond the filter, preventing my Ford F250 from starting?

All of a sudden, am having trouble starting my '94 F-250 Diesel. The engine cranks over but it won't fire.

I've verified that fuel is getting pumped to the fuel filter/water separator, which I've drained. The filter/separator is new, so I don't see how it can possibly be clogged.

How, what and where else can I check for a problem beyond that fuel and water separator filter?