It could be a lot of things. Lack of spark, lack of fuel due to a variety of failures. Are there any stored codes? How old is the fuel filter? What is the fuel pressure? If you spray some starting fluid into the intake and crank the engine does it briefly fire up, indicating you have spark?

A security issue is highly unlikely if it was running and died. If you can't hear the fuel pump prime when you turn on the ignition, before cranking, that's a good indication the fuel pump has failed. Check the fuel pressure or try the starter fluid spray like @chucktobias recommended.

I had a 2004 with the 3.8, those are pretty solid motors if taken care of. 

I doubt it's the Security because I had a backup key made at some Autoparts Store, and that never really worked all that well.  It did have the chip in it and could start the car, but about half the time it wouldn't start it.  It WOULD Crank it over, but then the red light in the dash quickly flashed "SECURITY" over and over until I stopped trying to crank it.  I didn't really care because it wasn't that expensive compared to modern key fobs, and I only really wanted it if I locked my keys in the car.

Air - Fuel - Spark - Timing.  One of these 4 are not working for the car not to start.  I seriously doubt it's timing as it uses coil packs instead of a distributor, so has to be one of the other 3 or a computer failure effecting one of the systems.

If the car hasn't been started in a while maybe some crud came out of the tank and clogged up an already old fuel filter.  But just for giggles check the fuses see if any of them burnt out.

My old Mustang mechanically was beautiful, only thing that went wrong  in 10 years with mine was the A/C Fan motor threw a bearing and died.  Other than that just maintenance stuff.