Eyes on the fries, baby
Are you gonna eat the rest of those French fries? Nonono—I'm not trying to steal your fries. In fact, I'm trying to save them for you. I know they're gross right now, soggy and cold, and you're tempted to throw them away, but please don't. In fact, if you're still out a restaurant, order some more to take home. (And if you ordered French fries for delivery, I salute your existence.) They're gonna be breakfast tomorrow, and you will be so happy about that. It'll take minimal effort on your part, and right now all you need to do is to stick them in the fridge—maybe blot them with a paper towel first to remove any excess grease or moisture so they don't get any gnarlier. Chug a glass of water (just operating off a hunch here) and go to sleep knowing that a most excellent breakfast awaits you in the morning. G'night.
Gooood morning! Did you sleep well? Glad to hear that. It's time for breakfast potatoes. Because you were so clever and thought ahead, you can have incredibly excellent hash browns, and plenty of the work has already been done for you. Seriously, you don't have to confront any actual potatoes at all. Now you can go about this a few different ways, depending upon your particular potato desires.
If you just want fries, easy peasy. Blot as much moisture as possible from the fries, and heat your oven to 400°F. Very lightly oil a baking sheet and heat the fries for five minutes, check them, and heat in five minute increments until they're crisped to your liking. If you're feeling particularly impatient or full of hubris, make a loose aluminum pouch, flip your oven to the broiler setting and keep a very close eye on things, lest your potatoes turn to ash and you are left weeping and peckish. A toaster oven can do if that's what you've got on hand.
You might also lightly oil a cast-iron skillet and get that pretty hot and flip the fries around in that for a while until they're to your liking. You could stop there, but if there's an onion around (or if you'd also ordered some onion rings and eaten most of the breading off, but saved the onions—but that's just crazy talk), maybe chop some of that up and add it to the pan. Got some peppers and tomatoes, or even some chunky-ish salsa? Toss it in and wait… waitwaitwaitwaitwait you just made home fries, you lucky dog.
Or you could skip the vegetables (keep the onions), toss in some random chunks of whatever meat is in your fridge, mess all that together and holy crap, there's some hash. And if there's a waffle iron in your possession, whoooo, are you in for a treat. Oil that sucker, layer in a fistful or two of fries, and cook until you have a patty crisped to your liking. Same deal with a panini press.
But whatever you do—seriously, lock pinkies with me and look directly into my eyes—never ever dream of using the microwave to heat up your cold, soggy fries. Down that path lays only further sog and sadness. You have been warned. My work is done.