If you press the "Start" button on your Onan generator only to find that the engine shuts off as soon as you release the button, this is the result of a serious problem. The control board is not passing along the required amount of voltage to the voltage regulator. The voltage regulator is the part of the unit that divides out power to whatever devices are currently plugged into the generator. You can confirm this by plugging a voltage meter into the regulator. If the meter is reading less power than what the gauge on the generator reads, this is your problem. Contact a certified electrician and have the generator repaired.
If your Onan generator isn't supplying power at all and the "output" gauge is at zero, this could be due to the slip rings on your voltage regulator. If these slip rings oxidize, Onan generators have a tendency to lose all output. It ultimately will not hurt the generator in any long-term way; it's just that your generator will not provide any power while this is going on. Simply purchase new slip rings at the retailer where you purchased your generator, unscrew the old ones and put the new ones on. Always keep your generator free from any water or condensation to prevent this from happening again.
If your display lights and indicator displays on your Onan generator won't turn on, check the circuit breaker on the back of the unit to see if it has accidentally been tripped to the "Off" position. If it hasn't, there is an electrical problem inside of your generator. The generator is simply not holding any power and has become damaged. You will still be able to use your generator for short periods of time, but not nearly for as long as you would if the model were brand new. Contact either the retailer or the manufacturer (at your discretion) and have your generator replaced.