There needs to be some residual magnetism in the field in order for the generator to start generating power. Sometimes that is lost and there is no power when the generator is started. You can apply 12V from a battery across the field winding for maybe 10 seconds to re-magnetize it. That requires getting at the wiring within the generator. A simple way is to just plug in a brush type drill, start the generator, hold the trigger and spin the chuck in the reverse direction. It should start generating power immediately and work after that.