The most common reason is a broken spring on the door. In this case the door will not easily or safely open manually either.
The springs do all of the heavy lifting. The opener just guides the door up or down. If your torsion spring is broken you should call a professional.
Garage door springs are extremely dangerous and if that is the issue with the garage door, please call a professional.
We can help – call us at (513) 821-5095.