If it is a Leece Neville it is likely a 2824LC but I cannot be sure. My unit had the same symptom, sat quite a bit over the winter, and when I started it up the voltage would not come up. I let it run a bit to see if the brushes are hung up to see if they might make better contact and start to generate the proper voltage. Hard to know what is wrong.
I had B+M in Pensacola rebuild mine and I think the charge was about $265. You will probably have an alternator shop in your area or you can ship it and have it rebuilt. There are other options and possible brand change, possibly a Bosch. My experience has been if the alternator quits working, it does not fix itself and it is time to rebuild it or replace it. Be sure the belt is in tact and all is spinning well back there.
Later Ed