Reiterating what ron-n-toni said about the switch/breaker on the generator itself. It has nothing to do with running the generator. The generator runs with it turned off or on. MAKE SURE THAT SWITCH/BREAKER IS ON.
On our 2007 PC's 4000 Onan, the breaker on the generator will trip by itself if I am not patient in waiting to allow the generator to run for 5 minutes or so before turning on the a/c the first time each season. I wait less than a minute thereafter.
I don't recall this, but like a house breaker, you might have to flip the breaker one way, then the other way to reset it to work. Then again, maybe not. I have not touched ours in 4 years now.
Gail and John have some good tips too. If the microwave is working, then the trouble is with the a/c, not the generator. If so, then it could be operator error with the a/c-furnace thermostat.