I also assume they are electric. If so are they on 1 switch or 2 ? more then likely 1,
*Next is there a motor at each leg or is it 1 motor with the cross shaft connecting them like the guys below mentioned. If it's 1 Motor with the shaft then they more then likely solved your problem for you. If not we need a little more info.
If it's individual motors...
1) Is the motor working and nothings happening, it would be something in the gearing.
2) If nothing is happening at that motor then we'll lean towards electrical problem.