Why is my microwave only lighting up?

Honestly speaking, microwaves aren't really user-friendly as far as repairs are concerned. Only a trained and well qualified technician should attempt any repairs on these machines. They're very dangerous, so please don't attempt any repairs if you are not a trained technician.

Please check the latch head (there should be two there locate close to the control penal on the inside face of microwave door). To me, there seems like a chance that one or both the latch head were snapped off or broken. Most microwave detacts if the door is open or closed with the latch head trigger a switch locates inside. If the door is open, the microwave will only light up inside but won't respond to any other operation. If that's the case, oder the part and replace it. (or if you can find the snapped off part on the kitchen floor, there is a chance you can glue it back)

If that isn't the case, you should then go and ask the profession for help instead. Good luck!