So it always cranks fine? is it a 3.9LMPFI? Do you have a tacho?
Start by looking at the tacho (if you have one) and see if it moves when you crank the motor. If it doesn't, unplug the TFI 6 pin plug on the side of the dizzy. Spray a bit of switch cleaner, WD40 etc. into this plug and refit it. Slide it in and out a few times and plug it in. See if it will start now. I could not work out what had caused my no start problem one day and notices the tacho not moving. Another indicator was the motor cranking slower than usual for some reason. I have only found one place to get these plugs from. It could be other things, but let us know what you find out ok?
EA Fairmont Ghia : 3.9L MPFI, 2.5" cat back sports exhaust, K&N panel filter. Pioneer head unit, pioneer 4" three way fronts, Pioneer 6" 3 way rears, 2x 12" subs, blue interior light.
My main pride and joy : '76 XB Fairmont GS, 250CI, 140,000 Km and all stock.... for now! (Factory air-con on the way)