having a serial number would be helpful.
but having said that, most of these units have a low and high speed lift switch on one side of the handle and low and high speed lowering switches on the other side, since you said you hear the motor humming? Is this without pressing the button on the handle? You may want to check those switches, remove the cover off the handle and check those, check the wiring and make sure nothing is pinched. it sound like one of the buttons are stuck, if one is that may cause an unexpected action like lowering because of a stuck button or two?
If that handle has been taken apart at any time in the past it sounds like it may not be assembled properly?