My Ford Explorer Sport Trac 2001 isn't starting,help?

You have to determine if you aren't getting spark or fuel first. Once you determine that you can figure out what the problem is.

Is the fuel pump priming when you turn the key to ignition? You should hear it hum for a second or two. If not, you aren't pumping any fuel. The fuel pump relay, computer relay(feeds the pump relay with power), the fuse, the fuel pump, or rarely the inertia switch could be the problem.

If you do hear it priming then it could still be a fuel problem. It could be the fuel filter, or fuel pressure regulator but since you already changed the filter and the fact that the regulator rarely goes bad I'd then look to the ignition system.

If you get no spark at the wire ends that plug onto the spark plug then you have an ignition problem. The most common causes would be the ignition coil pack or the ignition module. If you don't even get any cylinders firing when you're turning it over then it's probably the module but it could still be the coil pack.

Does the theft light go out?

Does the check engine light come on? key on engine off.

Do you have a spark? Fuel pressure?

Also, check the inertia switch under the dash. It's on the right side footwell area. If the red button isn't popped up then smack it several times until it pops up and then mash the button down again. Those switches give a lot of problems on Explorers for some reason.