Pendulums have traditionally been used to regulate time in mechanical clocks. On battery-powered clocks, they are for display only and moved by an electromagnet giving pulse to another magnet in the pendulum, which causes the movement. If your pendulum stops swinging or is swinging incorrectly, there are a couple of things you can do to start the motion again.
Place a spirit level on the clock to see if it is level. If the pendulum movement on the clock is not level, the pendulum will not swing correctly.
Adjust the level of the clock by adjusting the levelers on the feet if it is a standing clock or by moving it on the wall if it is a hanging clock. Check the level of the clock again with the spirit level to see if the pendulum swings correctly.
Change the battery in the pendulum movement. A low battery can cause the pendulum to swing irregularly. Check the pendulum movement once you have fixed the battery to ensure it is swinging regularly.
Unhook the pendulum from the pendulum movement. Check that it is not bent in any way; straighten out any existing bends. Rehang the pendulum on the movement.