2003 Pontiac Sunfire won't start

At the very least, you need to check for codes. That might point you in the right direction.

If you can't check for codes, do some of the basic tests. Have a helper crank it while you check for spark at the plugs. If you have the ignition assembly where the coils and ignition module set right on top of the plugs, it is a little more awkward to check for spark, but, it can be done.

If you have good spark everywhere, with the engine cranking, then check fuel pressure.

Oh, I didn't look about your theft system, when you turn the key on, meaning just when the dash lites come on, no crank, if the theft lite is on wait till it goes out before cranking. See if it makes any difference.