Key stuck in door lock
Don't laugh, this may work as I have used it on stuck or cross-threaded bolts and locks. Remember that heat makes metals expand, while cold makes metals contract. Looks like the usual 5-pin tumbler lock. I do not know how the key is stuck, but brass is a soft metal. After long term use the surface can becomes rounded and not move the tumblers correctly. The tumblers could be stuck from dirt or other contaminants. Try oiling the key first by forcing WD-40 or a light motor oil into the lock and tap the key lightly with a small hammer. This will normally loosen a stuck pin tumbler. Next step is to by freeze-spray and freeze only the key. This will make the key shrink a tiny bit. Wiggle the key and latch to try and jar the pin tumblers loose. Brass is not brittle when cold but do not use enough power to break the key. I am assuming someone did not play a dirty prank and put super-glue inside the lock. Even so, oil, freezing and heating will make the key loosen up, even if it takes a few hours.