Could be anything, it is a process of elimination, first check the posts by oldminiman, he has posted the diagnostic sequence to check for fault codes logged in the ECU, if this shows a fault, there is your problem.

If not it could be anything from dirt in the fuel system to the fuel pressure regulator, firstly, do you use the vehicle off road a lot, or in a lot of water, this is a likely possibility.

Begin be checking the fuel filter, ensure it is clean, break out the fuel lines at the rubber joints, blow all the lines including those that go to the tank, with compressed air, see what debris comes out. Check the fuel pump output by putting a rubber end into a bottle, get an assistant to turn on the ignition, the fuel pump will fire up for around five seconds, ensure pressure and flow are adequate. Connect all the hoses and prime the system by turning on the ignition for around ten seconds, then turn it off; repeat this untin the pump has primed the system, start the engine, see if it will now rev, if so you have cracked it, if not check the fuel pressure by fitting a fuel pressure gauge at the tapping point, this will identify no problem, or a fuel pump or fuel pressure regulator problem.

Remove the pump from the tank, if fitted in the tank, or remove the fuel pick up pipe from the tank. check this for perforations, also check the strainer on the end of the pipe for blockages, or damage, and if possible look in the tank for dirt or debris.

Finally, check the fuel tank breather for obstructions, it could be that the valve on the end is blocked or the internals have siezed.