It could be:

- Motor
- Flaky wiring
- Belts (maybe the belts finally snapped?)
- the bushings need to be greased a bit
- the signal to turn the motor isn't being sent

If you haven't shopped the domes in a while or at all, it's probably time. Make sure you remove the entire plate from the bottom with the hex head screws, and not the bolts for the plastic domes themselves. Pulling it out will probably reveal the problem.

Note: if they aren't turning, the likelihood that the left + right dome bulbs will burn out quicker is much higher. When I bought my F14 with non-turning domes, only the middle dome lit. 1683s aren't super cheap, so the time you spend shopping them out (an hour or two) is probably well spent, and you get the fantastic light show again to boot. Many of the parts from the assembly can be replaced with certain parts manufactured by John Deere (at the very least the belts), so you might even be able to visit a tractor supply for parts.