If the hole is open to the air, scrap the oven immediately and make sure that no one else can use it by cutting off the plug or removing the fuse.
Domestic microwaves use a frequency that will allow the radiation to penetrate the external atmosphere in the event of a hole within the oven chamber. This can be fatal to nearby bystanders.
If the hole does not pass through through to the exterior, you should still scrap the oven as the electronic components will become damaged due to the microwave radiation.