Craftsman Garage Door Opener

Hello, I have a Craftsman Model 139.53970 Garage Door Opener that is not working. When the button is hit the chain tries to move but cannot. There is a buzz/hum for one second then the power is cut and a green light on the back starts flashing. I have disconnected the door from the opener and can open and shut the door manually with no problems. It almost seems like the motor is out of phase...like it thinks the door is up when it is actually down.