With one leg working and the other not, it is almost surely a sheared pin (bolt), or one that fell out. Probably a 1/4" or maybe 5/16" bolt. There should be a shaft going across from one side jack to the other. Look on the end at the jack that is not working and see if there is an empty hole in the shaft/coupling. If so, replace it with a grade 2 bolt of the same size.
If there is no bolt missing, the problem may be in the gearing for that side. You can have some one actuate the switch while you watch that side. If the shaft into the gear box is turning and the jack is not moving, then a problem in the gear box.