I have a Craftsman riding mower Model # 917.270930 Mowed yesterday and shut her down. Went' to use it again about an hour later and the cluch will not engage. I can put the forward and reverse shifter in gear and pull the mower forward and back and the gears engage the engine. I can move it by hand in neutral. The brake pedal goes all the way forward and returns to the engaged position but no clutch..... What is going on ?