I closed my microwave door, and for some reason the microwave started, and now won't shut off. I also can't open the door.

Check three door sensors, especially for two closed state switches.

You may have a problem with a door switch or door switch mount, or there may be a shorted relay on the control board, which can easily be repaired.
Our help file on door switches is here, and the Advantium 120 service manual is here.
You can order GE parts from the Pros here.

see this causes and fix it. God bless you
Open Lever If the microwave door won't open the open lever might be broken. This part can be replaced separately.

Door Spring If the microwave door won't open the door spring may have broken. The door spring is located inside the door and keeps downward pressure on the door hook.

Door Latch If the microwave door won't open the door latch might be broken. The door latch is inside the microwave. The door hook, which is part of the door, engages with the latch to keep the door closed during operation. If the door latch or actuator breaks the door may not open.

Door Hook If the microwave door won't open the door hook might not be releasing properly. The door hook is installed inside the door and normally releases when the door is opened. The hook is made of plastic and will break over time. When this happens it will need to be replaced, it can't be repaired.

Button Spring If the microwave door won't open the button spring might be broken internally. The button spring can be replaced.
Button If the microwave door won't open the button might be broken internally. The button can be replaced.

Handle Actuator If the microwave door won't open the handle actuator might be broken. This is a plastic part which can become brittle over time. This part is connected between the door button and the door latch assembly.

I have the same problem. Is the light still on inside the microwave? Mine is on and the microwave thinks the door is not shut. I took the top off of the microwave and the latch needs to push down on a switch once shut. The latch was reaching the switch, but I think some spring in the door, pushing the latches down, has broken since it reaches the switch when closed but there isn't any downward force to flip the switch.
I am trying to figure out how to repair the latch but if your light is on when you "shut" the door, you may have the same problem.

check the hooks and door release button and repair or replace, when this is broken, the switch is not close the microwave will not START.
SEE the diagram
God bless you

It sounds like one of the door switches has become defective, each must be disconnected on the one side and tested with a digital meter using the ohms scale.

It is one of two things, a defective door switch, or the keypad control board has failed.

Yes! this should be checked right away to make sure the magnetron is not running with the fan when the door is open , it may be a problem with one of the door switches going bad, left unchecked it probably blow the fuse in the microwave killing all power, but it is quite repairable. But please do not use until it has been checked out by a professional for safety reasons.

The door switches are what tells the microwave that the door is opened or closed. If they play up they could be causing your microwave to act strangely.

MICROWAVES ARE MADE TO BLOW THE FUSE IF THERE IS A POSSIBLY OF DANGER--THAT'S A GOOD THING...SAFTY SWITCHES ARE MADE TO CLOSE(SOME UN-CLOSE) WHEN THE DOOR IS OPEN AND SHUT AND IT SOUNDS LIKE YOU DO HAVE A LITTLE DOOR PROBLEM---THE REPAIR SHOULD BE FIXABLE AND MAYBE REASONABLE,,MY GUESS COST-WISE ,,,75.00,,OF COURSE ,IT ALL DEPENDS ON THE SERVICE COMPANY--TRY AN INDEPENDANT SERVICER (NOT SEARS,ETC) GOOD LUCK TUNICA