Flashing the field is the first thing to try. That happened to my Northern Lights
one time. It cost me $250 to have someone come out and do it, and he said he felt bad because I could have done it myself for nothing.
The directions on how to do it are in my manual and after having watched someone do it, I would try that before I spent any money
He explained that generators just lose their magnetism some times. He said he had seen diesel
generators on oil
rigs run for years. Then one day they turned it off and it lost
the field and then it didn't' do it again for several more years after he came out and flashed it for them.