PTsideshow, are we talking about an anniversary clock? Is it a quartz movement? If so, then the spinning motion is an added mechanical feature and works totally independent of the time keeping. I just fixed my grandma`s about 3 months ago, it had been keeping time for years but the motion of the pendulum balls had stopped. The main cause is that the suspension wire has broken but it would be obvious as the pendulum won`t feel free to spin. If your lucky, it`ll be a simple case that the fork has become lodged incorrectly in the mechanism.
If you remove the back below the battery, you`ll be able to see what is going on. If your lucky, you`ll see a stamp on the movement and will be able to get a replacement with suspension for not too much money. I got mine off ebay, a direct replacement but I was lucky because they aren`t in production any more.
If we are not talking about dome clocks / anniversary clocks then I`m sorry and ignore me!