We are really far away from a diagnosis.
#1 - Pull codes. Search the site on how to do this, it is the same as the 5 liter cars.
#2 - Report back. At this point, it could be literally anything...
Worn spark plugs, failing ignition coils, failing ignition module, weak fuel pump, clogged fuel filter, bad fuel injectors, bad MAF, contaminated fuel, plugged exhaust/clogged catalytic converter, ECM...
Cranks OK, but No Start Checklist for Fuel Injected 5.0 Mustangs model years 1986-1995 A word about this checklist before you start: it is arranged in a specific order to put the most likely failure items first. That will save you time, energy and money. Start at the top of the list and work...
Start with the checklist that general kartheif recommended first if it cranks but wont turn over. If it doesnt even crank then my money is gunna be on a bad or heavily corroded starter hot line causing an incomplete circuit.... visual inspection and voltage testing would make your life alot easier.