Garage Door Trolley Jammed

This can be a quick fix by identifying the object and then removing it.
Garage door trolley jammed. The trolley can become lodged against the stop bolt tab because the door has either opened or closed too far. If there is something in one of the tracks obstructing the free movement of the rollers the garage door can stick open. Typically this is caused by a mis adjustment. Any problems with springs or pulley should be examined by a garage door professional.
Don is correct in that you may have a stripped drive gear but you will need to remove the motor cover to see if that is the problem. This video from sears partsdirect shows how to troubleshoot motor and travel problems with your garage door opener. You will want to start by bringing the adjustment close together so the door goes neither all the way up or down and then slowly take them back a little at a time trying the door again and again. Your jammed garage door can be caused by something as simple as an obstruction in the track.
Check to see if anything a shovel broom etc fell and is now stuck in the track that is causing it to be lodged. T his is a pretty uncommon problem but one that can happen after you ve first installed your garage door opener. The image below will show you the front stop bolt. This is related to the up limit switch.
Try making small adjustments. It s important to get this switch at just the right distance from the motor so that the door is able to close all the way but the motor also knows to stop running. Jammed or obstructed track your garage door works because the garage door opener unwinds or winds the torsion springs pulling or pushing the garage door with rollers along a track. A pulley can become jammed by an obstruction or possibly a misaligned or hung up cable.