Engine belt

Well I was driving the other day and just as I pulled up to a light the steering wheel kind of locked. So I stopped at the light and could barely turn the wheel. So I made the left hand turn going over a bridge and I notice it did not get easier to turn as I put on the gas. So I went on across the bridge and turned right drove half a block and parallel park the van as best I could, got out and checked power steering fluid. It was really low but it was so in its tank. It did not read on the cap meter thing built in. And that is when I noticed the belt was off the engine. Because this van the engine sits sideways. What caused the belt to just come off? Or what is the problem?