If you wouldn't mind sharing, what's the brand, model, and part # of microwave? Can look up parts and repair manuals, it may be easy and cheap to replace 
, especially if it's the magnetron cover, you just unscrew and replace. If it is the magnetron cover, you can still use the microwave, but meantime, cover your food with a paper towel or something to keep splatter from getting there. If it's not the cover, and a cover for something else, then further investigation is warranted before use. Qualifications: I have broken many appliances trying to DIY fix them, and successfully repaired a few, too. Batting average improving!
It may be possible that, if this is the magnetron cover, it arced from the inside, too, which would mean that part has to be replaced, or the whole microwave. Hard to tell without seeing/knowing more.
