Alternator Full fielding..??

Assuming you have a a isolated field alternator (typically square back in design and 2 field wires)
One of your field wires is 12v from ignition switch and the other is a variable ground from the voltage regulator.

If the wire from the voltage regulator gets grounded you will have the situation you describe.

A quick check is to see if there is 12v to ground to one field wire at the alternator while it is running and the other field wire to ground should be other than 0 volts like 5 to 10. If it is 0 volts to ground then disconnect it from the VR and test again at the alternator. If it is still 0v then that wire is shorted to ground somewhere along it's path.