I have a Liftmaster garage door system. This morning the door only opened to 85 percent. I closed the door using the wall switch and the door closed completely. I then tried to reopen and the motor hums but the door won't open. The motor continues its opening sound but the door does not go up.

I then detached the garge door and opened manually. Once fully opened I activated the garage door opener using the wall switch and the motor hummed and the chaines did not move? I don't know what to do next. What could the problem be?

Chain is off the drive sprocket or the gears inside the motor are shot.