A whole bunch of things....does it crank and no start ? If it's just cranking do you hear the fuel pump kick on when key is turned forward. If you don't check the fuel cut off switch in the trunk to the drivers side of the latch. If the red button is popped up press it back down. Check the harness connector between the rear bumper and the fuel tank. That's the connection for the fuel pump. Unplug it and plug it back in. You can also take something small like a pen tip and find the valve on the fuel line between the alternator and valve cover area. Take the cap off and push the valve...fuel should spray out. If you don't spray fuel your pump ain't working. If it does you have another issue....like spark.
I have a checklist for a car that won't crank when you turn the ignition switch to Start for 79-95 Mustangs.
I have another checklist for 86-95 EFI Mustangs that crank OK but the engine will not run.