Generac 20kw/999 Guardian generator, all automatic computer controlled, ran for two hours then shut down. Red light is on. Drifting snow piled up around generator. I think it overheated and shut itse

SOURCE: Generac Guardian 17.5KW Generator dies

If this is one of the big "taco cart" portables you need to check that the vent on the gas cap is open when you're running the engine. In the center of the gage on the gas cap is a little knurled knob, turn it counter clockwise (lefty loosey), about a half turn or so and run the generator.
If that doesn't solve your problem add to it here, otherwise please give me a rating I'm trying to build a little credibility here and could use the help.
Carl

SOURCE: my standby generator is just cranking but wont turn all the way

If it was installed only five months ago, it should still be under warranty. Call Home Depot and have them send out a repair person.

SOURCE: Attempted to start Generac Guardian 15 kw air

I work on these every day. Its either gonna be the rotor or stater, the voltage regulator, but most likely its a bad stepper motor which is the throttle motor, or control board. Thats all that could be causing that so its one of those. If its the rotor or stater thats bad it causes the generator to RPM up because it thinks its not producing enough power due to that. I had the same problem with the same one we just fixed and it was the slip rings on the end of the rotor so bad rotor.

SOURCE: 10kw Generac Guardian not cranking, shows rpm sense loss

Sounds like Generator mode switch should be "ON".

SOURCE: Generac Guardian 15 kw overspeed fault

Did you remember to set the overspeed shutdown threshold on the control board?