DIAGNOSIS That F0 code is the microwave's way of telling you the door switch system has a problem. One of the little safety switches that tells the unit the door is properly closed has likely failed or gotten stuck.
FIX First, unplug the microwave for safety. You'll need to remove the outer case to get to the switches. Take off the grille at the top, then remove the screws along the top and sides to slide the main cabinet forward. You'll see three small door switches mounted near the latch. Use a multimeter to test each one for continuity when you press the door latch button by hand. The one that doesn't click or show a change is your culprit. Replace it with the exact GE part.
IF THAT DIDN'T WORK If all three switches test fine, the issue might be in the wiring harness connecting them to the main control board. Check for any pinched, frayed, or disconnected wires along the path from the switches to the board.
HEADS UP Be extremely careful when handling the high-voltage capacitor, even with the unit unplugged. It's the big silver can, and it can hold a dangerous charge. It's best to discharge it with an insulated screwdriver across its terminals before you go poking around near it.