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

All of a sudden, am having trouble starting my ’94 F-250 Diesel. Engine cranks, but it won’t fire. I’ve verified that fuel is getting pumped to the fuel filter/water separator, which I’ve drained (it’s new, so I don’t see how it can otherwise be clogged.) How/where else can I check for a problem beyond the fuel/water separator?