Most likely one of:

If it is a basic generator it may be the capacitors or diodes that have broken down.

If it is a 'cheap' Chinese generator then it may well be the AVR.

Take the end cover off the generator and have a look. If there is a large lump of plastic with a load of wires going intoit then it's an AVR.

If it is a large generator then it could be that the rotor has become de-magnetised.