Pizza is one of the most common leftovers around, but it's also notoriously difficult to coax into a reasonable facsimile of its former self after a couple nights spent in the fridge. Sure, you can pop a slice in the microwave or toaster oven in a pinch - some people just eat it cold - but the end result will usually be soggy. If you really want to get as close as possible to restoring pizza to its fresh-from-the-pizzeria glory days, there's a technique that you should always have in your back pocket.
If you want your reheated slice to get its crispy crust back, you need to take a couple cues from professional pizza ovens: They get really, really hot (sometimes past 900 degrees), and their deck (the floor where the pizza sits) is also really, really hot. So instead of putting a cold baking sheet into the oven, keep the pan (or even better, pizza stone) in the oven while it's pre-heating to 450 degrees F (or 500 if your oven can handle it).
After it's done preheating, pop the pizza onto it, let it get nice and hot and bubbly in the oven for about 10 minutes, then let it rest out of the oven (still on the hot baking sheet) for about 5 minutes until it cools slightly.
The end result will never replicate the pizza that it once was, but the crust will be crispy, the cheese will be melty, and it'll definitely be tasty. Or, you could always just say "forget it" and head out to try the best pizza in your state.