Im riding along on my trainer and I shift into the lowest gear...53-12 I think. I said up to ride and all of a sudden it freezes up on me. At first I thought it was the trainer, it over-heated or something. So I took the bike off and when I turn the crank the wheel wont spin, like something is holding it. I thought maybe I bent it out of true but when I flipped up the brake it still wont spin. I can spin the crank backwards and the gears spin but it wont spin forward and spin the tire. Anyone have any idea what the hell I just broke?