To fix a Ford F250 that doesn't start, first identify the root of the problem, such as a dead battery, faulty spark plugs or low fluid levels. The gas gauge may also be faulty.
Insert the key into the ignition switch, and turn it. If the dashboard lights turn on but the engine doesn't turn over, open the truck's hood, and have someone turn the key as you listen for a clicking sound from the starter relay. If you don't hear the clicking sound, check whether the generic electronic module is in good condition. Additionally, inspect the power connections to the starter relay. If the engine emits a clicking sound, the battery has a low charge.
Remove the spark plugs from the top of the engine, and check for signs of oxidation. Faulty spark plugs are dry or have black spots at their tips. Replace the faulty spark plugs. Next, check the levels of the oil, the transmission fluid and the coolant. If any is of the fluids is below the required level, refill.
Reset the fuel pump's Off switch, which is located in the access panel next to the passenger door. Press the Reset button, and attempt to start the truck. If the F250 still doesn't start after these steps, consult a professional.