Presuming that you have iliminated the possibility of any metal objects that are not meant to be in the machine being in there (and this includes dishes and other items with silver or gold leaf on them, which WILL spark and burn!) then take a look inside the oven its self. You will find a waveguide cover. It is usually a strange flakey silvery plastic/card like material called Mica and is usually on the same side of the microwave wall as the control panel (So if the picture on this thread is accurate, I will guess the Mica cover is on the right hand wall inside as you look at the machine from the front) . Over time, the magnatron (the thing that creates the microwave energy) will often burn a hole through the mika, or cause a dark brown or black burn mark in it. If this cover is even slightly damaged then it may well cause sparking inside the machine. These covers are designed to be removed and you can buy replacements for them from electrical stores.
It is possible that the unit takes in too much current to cut off the controller unit so if there is spark please do not switch on unless you have confirmed the fault. It is possible that the Magnetron is leaking and sparking to the ground. As you have mentioned it was stored, it is quite possible that insect can get into the oven and create soft spots. When you switch on these will create a short and make the voltage jump and in the long run damage the magnetron.
So Remove cover after disconnecting , check for dirt/moisture in the cabin, if so wipe clean and dry, check the magnetron and area clean and use an jet air to clean off.
Now keeping clear switch on , use a cup of water inside to test.
If you notice any sparks shut off as the magnetron or the main HT capacitor can be faulty. replace with same type.
If there is heating without any issues , you can use the microwave safely
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