What causes a generator to not produce any power? How can I fix it?
Why does your generator not produce any power / voltage?
The most common causes of a generator producing no power are the loss of residual magnetism in the alternator and a failed AVR or another excitation component. If the voltage is around 50-70V on each phase, then it is likely the AVR needs replacement. if the voltage is 0-5V per phase, it is probably a loss of residual magnetism.
If your generator doesn't have a PMG system, then it is almost certainly a a self-excited alternator. These rely on the residual magnetism of the alternator, left from it being run previously, to build up its initial voltage.