90 F-250 backfired twice and quit

I have a 90 F-250 which while driving on the highway backfired twice and then stalled. It is still cranking but won't turn over. Any ideas what this could be? Thanks.

Sounds like it jumped time. I had a 95 Mercury Mystique that I was driving home and it lost power, it would not turn over with the ignition etc. I poped the clutch got it runing again but still had no power. When I got it to a complete stop it wouldn't start at all. Ended up having a friend pull me home as he was pulling me I popped the clutch and heard a loud boom, it backfired and at the same time blew all of the breather apart. Had a mechanic look at it later, he told me it'd be about $1500 to fix it because it jumped time.