My chain saw seized up on me. I was sawing through a small log and the chain just stopped. The engine would still run, but the chain would not move. Thinking the chain must be pinched or something might be wedged in the chain, I took the chain and bar off, but the sprocket that the chain fits around will not turn either. I started the engine again, but that sprocket will just not move. What could be the problem? I just bought the saw in January.