Microwaves, at least the cooking circuit, tend to be very simple devices.
You have a transformer (very high voltage output) A Diode, a Capicator and a Tube (Magnatron if I've spelled it properly).
Odds are it's the diode
But it might be the device that turns the transformer on and off.
Open the door, check the nameplate on the microwave itself, Mine says LG and frankly.. I would not bother fixing an LG
There is one and only one thing "Special" about mine (Well technically 2).. The one, is what LG would call an "After market modification" Some sheet metal, screwed to the case over the hot air exhauxt port to direct the high mositure hot air to where it needs to go (In my case out the front of the mounting trim) This can be added to any Microwave (Heating and cooling shops do that kind of metal work)
The "technical" special is the Dometic paint job (Different insert in the door says Dometic instead of Lousy Goods, er, LG.)
Cheapest option if you are NOT an electronics technician (I am) is to replace it, they are not worth the cost of repair, epically this Friday. Just get one teh same size and if you are lucky the same ductwork will work, if not have a heating shop bend a bit of sheet for you.
Home is where I park it!