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.
If you have low voltage on your alternator, read more about low voltage on an AC alternator for how to diagnose and fix the issues.