I had the same problem. It was caused by the upper bearing in the gearbox being rusted to the point the drive shaft could not rotate.
If you remove the motor and the gearbox, and you can't turn the shaft the motor turns, then it's likely the bearing. It's fairly easy to replace. Take the cover plates off both sides and you'll find an oil seal and a bearing below each cover plate...these will need to be replaced.
There have been several discussions on when to replace the large bronze gear, have a look at those as well.