It could be several things, cps, fuel pump, glow plugs, hpop, IDM, PCM.
First of all check your fuses could be as simple as that.
Next check the CPS(Genuine Ford 7.3L Diesel Cam Position Sensor (CPS) - Blue
) This sensor going bad can cause a whole plethora of strange problems for the 7.3 and is cheap/easy to change as a first measure
Next best thing to check is if your glow plugs are getting power ([Troubleshooting GPR & GP's
Then next check your fuel pressure it should be if i remember correct AT LEAST 35 psi(could be wrong if someone wants to correct) to even fire and 65 psi is optimal for a stock truck.
Next find another working IDM from any truck from 94-03 7.3, they are all compatible then swap it in to see if it starts. That will tell you you need another IDM
Next would be to check your HPOP reservoir oil level. It should be within an inch from the top of the reservoir check port. That would signal a weak hpop/lpop. Best way to test is if you can get your hands on a scanner than can read live data from the truck. The HPOP needs to be at least 500 psi to let the truck fire the injectors.
Thats just a few of the common causes for no start, by no means is it 100% but it will get you a start