Did you put a new battery on it? It sounds like the battery is dead. If not the battery, look at the alternator. IS the 12V out put before or after the regulator. After, it should be putting out a bit over 14V. Before the regulator it should be putting out a lot more voltage that that. The rotor may have lost it's magnetism. It can be remagnetized at an automotive electric shop.
But it could be a variety of other things, so you just need to go through the charging systematically.
Bare