I have 2 trail touring (two-ups)that I keep around for kids to ride and play on. Last weekend we went riding in the mountains and one of the trail sleds had an issue. After taking a break we started to head out and the track would not move. The engine would rev up and the clutch would engage, but the track would not move, at all. We removed the belt and towed the sled back to the sno-park. The track never broke free and remained locked up all the way down the mountain. What caused this to happen??????