Could be a loose wire, bad connector, burned out filaments, or the high beam switch. My guess would be the high beam switch but you really should take some readings before your simply start replacing parts.
I would pull one of the center lights and see if the low beam connector is getting voltage. If not, its more than likely the high beam switch. FYI, I had two low beam filaments go out at exactly the same time on my old F-100. I figured there was no way they could go out at the same time so I replaced my highbeam switch only to find out it was the filaments. Bottom line, it can happen.
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.
Last time it happened to me was my annual club registration inspection... no high beam until I hit the switch a couple of times... the inspector says he sees it all the time... old cars not driven much, rarely at night, so dust/gunk gathers on the contacts.
