Before you remove the microwave casing, you need to read up on how to discharge a high voltage capacitor, i'd explainb it here but i feel much safer if you were to look it up and fully understand it before attempting any repairs as the high voltage capacitor can cause fatal injury and/or death.
Ok, after reading up about how to discharge a high voltage capacitor, this is what you need to check out.
Step 1, remove the microwave casing and discharge the high voltage capacitor to prevent injury or death.
Step 2, Look for the fuse which you may find in a small plastic case with wires coming from each end, one end leading to the high voltage capacitor and the other end leading to the transformer and the magnatron assembly, the magnatron, or maggy, has a cube-ish shape and contains magnets which are visable through the grill. You will be able to see with the naked eye
if this fuse is faulty as it will look like a blown fuse, if so, replace the fuse.
Step 3, At this point if you have a spare fuse you can VERY CAREFULLY plug the unit in and VERY CAREFULLY test it to see if it heats, if it does heat then VERY CAREFULLY unplug the unit, discharge the capacitor, and replace the case. If it still isnt heating you will need to replace the magnatron assembly, contact your authorised repair agent or the manufacturer or the microwave for directions to spare parts. Be very careful with new magnatron assemblys because if you drop it, you will damage or fracture the magnts within and render it useless.
I hope this helps. Good luck