D3400 DIESEL ENGINE Lost Fuel Pressure

We had the diesel engine running what we thaught was perfectly well for 2 different times-20 minute sessions ,good fuel pressure,good oil pressure ,good engine temperature ,left it for3days tryed to start it again and no fuel pressure,I checked the overhauled pump with the spring, plunger,gaskets,seals everything was in place.I drained the fuel filter housing which I had cleaned before all clean fuel ,took the fuel filters out everyhing clean,can anyone tell me what to do next

Check the flow from the tank. There might be something floating around in the tank the will intermittently block the flow. Also check the fuel tank cap that the vent is working.

its possible some crud has been loosened in the tank and this has now blocked outlet strainer. did you clean the tank. also have you serviced the lift pump.
clean the tank and try again. maybe the fuel tap has vibrated closed check the tap gland it might need fresh packing.

That's on a Gen Set. right? Is the fuel being lifted from a skid mounted tank? Check for air leak in the lift piping. Is there a check valve in the piping? If you suspect compression leaking into the injector overflow line disconnect the line and hook up to a hose in a bucket of fuel to see if there are air bubbles when running.