Could be the filaments of the low beams both burned out, as hokie say's it happens, but I would suspect your high-low switch. Coincidentally I have an F250 , the high beams went out just as I was rounding a curve on a winding road in total darkness. It was the high/ low switch. They are exposed to the elements and tend to corrode. It could be the wiring harness but let's eliminate the most common fault first.