What else is going on? Do you have power to the lights, horn, other accessories? Installing headers involves getting "up close and personal" with the starter electrical connections. Perhaps a wire has been damaged. Make sure all connections to the starter are intact. Remember that headers have earned a reputation as "wire burners", and the reputation is well disserved.
Does the engine crank over?
Not knowing what year that Jeep is or what engine it has, start with replacing the fuel and air filters.
To get the 2009 NPR started after replacing the fuel filter, simply start the engine.
Start by replacing the Engine Coolant Temperature Sensor
Try replacing the engine coolant temp sensor.
Diagnosis: Engine Won't Start or Run Check related link below
no just go ahead and start engine
as long as the belt is on right it will work, start it
Try a new battery, or get a jump start. Sometimes there is not enough power in the battery to start the engine.
Check engine light, low power, no start, runs rough, bent valves,Check engine light, low power, no start, runs rough, bent valves,
try replacing the delta pressure feedback sensor. this fixed my explorer.Or you can try replacing the relay.
Do it wrong and you will suffer severe engine damage the instant you start the engine. So leave this repair up to a professional.