1999 Mercury Tracer Timing belt

My engine quit,I looked at the timing belt and it appears fine. however the number 3 spark plug gap was touching the electrode, the number 4 spark plug was crushed into the electrode. I think the cam shaft sproket may have sliped as my timing marks don't line up. Even after I re align the timing marks the engine won't start, i have a spark. Is it possible for the timing belt to stretch, or slip over some of the teeth.