You Should Never Run Your Microwave Empty. Here's Why
You probably know you shouldn't microwave aluminum foil, since it sparks, and you don't want to microwave a whole egg, lest it blows up, but what about microwaving nothing? That surely isn't bad, right? What's going to happen? It turns out that running your microwave while empty (which is most likely to happen by accident, when you forget to pop whatever it is you want to cook actually into the microwave) can be quite dangerous.
Microwaves emit micro waves via an electron tube — the waves reflect around the appliance interior, where they're absorbed by your food. Normally, those waves heat and/or cook your food as they cause the water molecules within a food item to vibrate and create heat (via the FDA). However, with no food in the microwave, there's nowhere for the waves to go and so they're reabsorbed by the appliance. This results in, at best, damage to the microwave and, at worst, a kitchen fire (via The Daily Mail).
What to do after you run your microwave empty
So, let's say you've already used your microwave while it was empty. What should you do? According to GE Appliances, if you've only run your empty microwave for a short amount of time (fewer than five minutes), you may be fine. No big deal. If, however, you've run your empty microwave for a longer amount of time, it may very well overheat, emit a burning smell, and then shut off on its own.
If this occurs, you'll want to allow the microwave to cool down, before resetting it. Even after that, though, you may find your microwave is permanently broken. You'll want to give it a test run (with something inside) to ensure it's heating and that the electron tube isn't permanently damaged. Hunker recommends heating a microwave-safe container of water in the microwave for a minute — if the water is hot, the microwave is fine. If not, you need a new microwave.