Use the microwave if there is one in the room. Some hotels have microwaves in common areas, for example near a market/pantry outlet that sells snacks. Policy differs whether hotel guests can use these microwaves for food not purchased there, so be sure to ask first and don't make a mess or consume the pizza in a common area.

Campus legend says that irons can be used to cook food. I know they work for things that otherwise would be fried without fat or grilled, such as steak, but it can make a mess of the iron.

Hairdryers are good for drying clothes in a pinch, but I don't think most of them are hot enough to make the food hot rather than vaguely lukewarm.

If the hotel water is really hot, you could experiment with putting the pizza in something waterproof, such as a 3-1-1 storage baggie, and immersing in very hot water. You could even try the same if the hotel room is equipped with a kettle or even a coffee maker that can be used to heat water for tea, and similarly if you travel with an immersion water heating coil.