Liftmaster door opener, motor hums but door does not move up or down!

I had the same problem. My garage door was in open position, when I press the button to close it, all I hear was hum noise for 2 seconds. What I found out the problem is the metal piece (Trolley) that door arm attach to was in contact with the stop screw on the rail. In Chamberlain manual that piece is called Trolley. Door Arm is attahced to Trolley and can be released from Trolley by pulling the rope. There is a big screw on the rail that stops trolley from going further towards the opener housing. You should leave some space between the screw and trolley. Once I made the adjust, it works. I have a photo but I cannot post it here. I had the same problem. My garage door was in open position, when I press the button to close it, all I hear was hum noise for 2 seconds. What I found out the problem is the metal piece (Trolley) that door arm attach to was in contact with the stop screw on the rail. In Chamberlain manual that piece is called Trolley. Door Arm is attahced to Trolley and can be released from Trolley by pulling the rope. There is a big screw on the rail that stops trolley from going further towards the opener housing. You should leave some space between the screw and trolley. Once I made the adjust, it works. I have a photo but I cannot post it here.