Light and fan stay on and turntable no longer work

The switch for the turntable does not disengage anymore. Could be a relais, could be an electronic switch. When opening the door, a second switch, in series with the other one, connected to the turntable motor disengages, stopping the motor. The light is switched on, because you opened the microwave. Perhaps you never noticed this, but it is the normal way that should work.

sounds like a cross wire, and a short. I'd inspect the socket for the bulbs to ensure that there's no short circuit.

you have a couple bad switches

Some GE units were starting on their own. I would definitely unplug this oven and call GE to make sure yours wasn't one of the ones being re-called.

Inside there's a little door switch there's 3 of them they have to be in sequence You can adjust them with a screwdriver phillips

Most likely you have a magnetron problem. Easy to replace (DYI) but make sure the high voltage capacitor is fully discharged first against the case.

Don't have a fix for your problem, but my Frigidaire microhood just did the exact same thing--about the same age, started cooking some food and it worked for a few minutes and then just stopped--time kept going, light stayed on, but turn table stopped and fan noise cut out. I tried it a few more times and it seemed to fade in and out and then stopped trying.

OK, you have two components that are faulty. the magnetron and the turntable motor as well. replacing these two components will fix both issues.

Possibly the light bulb has blown, which has blown on a track on the board which has taken out the turntable motor and the fan. Also, there could be a problem with the door switch.