Hi caroline. Well thats not good. I believe that the MAGNETRON . Its a special tube that makes the "microwaves" check this out on google.com type in "what makes a microwave oven work?"
Just buy a new one it will be cheeper than repair.
They have high voltages and dangerous radiation inside - or rather they should have - yours probably does not.
Microwaves are not generally repairable by the user - too complex/dangerous - needs special test equipment.
Here is a picture of the bit that sends out the microwave energy...