The power went out on the microwave. We were cooking potatoes in there, and it just went dead. We were using the Jenn-Air oven below it at the same time, but that is still working.

There's a circuit breaker behind the microwave. Yup, that's where
they put it, believe it or not. The microwave and the oven are joined
together so you have to pull both of them out to get to the circuit
breaker. There are screws that hold the microwave and oven to the
cabinet it's mounted in. There should be 4 around the microwave
door and 6 around the oven door. The oven door needs to be
removed to get to the 2 bottom screws. The oven door has a metal
tab on it's 2 hinges that must be pushed down to unlatch the oven
door hinges from the 2 (per side) posts that the oven door pivots
on to open and close. After the door has been removed you can
take out the last 2 screws, you should also remove the vent at
the very bottom of the oven by removing it's 2 screws. Make sure
you have a least 2 strong people to pull the whole unit out, it's
also a good idea to have something sturdy that's the same height
as the bottom of the oven to rest it on after you pull it out. You
don't really have to pull the unit completely out, but it helps. You'll
see the small circuit breaker on top of the oven at the rear of the
microwave, just press it to reset it. Putting the unit back in the
cabinet is the reverse of the removal, but be sure you don't pinch
the armored conduit that contains the gas line for the oven. It'll
probably want to fall behind the oven as you push it back into
place like it did at my friends house. What we did was I pulled it
up and held it at the back of the microwave while 2 guy's pushed
the unit back in. Fortunately, it stayed in place after they pushed
it part way in and I had to pull my arm out of the way. It's a really
crappy place to put a circuit breaker for the microwave. I mean,
come on! You couldn't put it under the microwaves bottom cover
plate? There's room for it there! Anyway, that's how it went at my
friends house. I hope this helps you fix you're problem.

In some microwaves there are overheat sensors, if it does not work after it has cooled down do not attempt to have a look inside unless you have ben trained on microwave repairs, in some cases the unit can hold a enough of charge of electricity to kill you.

take it to a trained engineer...if it's a cheap one...just buy another