How do I fix a cast iron pot that was heated empty for hours?

Your pan is NOT ruined.YAY! What has happened is that you have burned off most of the seasoning. Any other kind of pan would be ruined, but your cast iron pan only needs to be stripped and reseasoned. If you have a self-cleaning oven cycle, that is a great way to completely strip the pan. You can also throw it into a hot fire (like a fireplace, wood stove, or campfire if you've got one) and just leave it until the fire is out and you can touch the pan. For any of these methods, the pan should be up-side-down.

Another thing you can use to get to the bare metal is oven cleaner, in a hot oven or not, depending upon the product. Again, the pan should be up-side-down.

Complete the mission with steel wool and dish soap, an SOS pad would be great for this.

Now it is naked, and needs to be seasoned. Unfortunately, it will stick like hell until you've got a good solid season. Season it a few times and cook only greasy food in it for a while. It'll come back eventually.

You may get answers that say you don't need to strip the seasoning you've got left. I would disagree with that, seasoning over badly damaged seasoning never gets as strong and smooth. Start from scratch.

Look here for seasoning advice: What's the best way to season a cast iron skillet?

I have recently become a flaxseed convert, so my preferred method is this one, from Sheryl's Blog, which I first saw thanks to Neil G's answer to the above Seasoned Advice question. The whole article is worth reading, but here are her instructions for seasoning from scratch. I just did this to one the cast iron pans I inherited from my grandmother. The result was fabulous.

The Recipe for Perfect Cast Iron Seasoning

The basic idea is this: Smear a food-grade drying oil onto a cast iron pan, and then bake it above the oil’s smoke point. This will initiate the release of free radicals and polymerization. The more drying the oil, the harder the polymer. So start with the right oil.

Go to your local health food store or organic grocery and buy a bottle of flaxseed oil. It’s sold as an omega-3 supplement and it’s in the refrigeration section because it goes rancid so easily. Check the expiration date to make sure it’s not already rancid. Buy an organic flaxseed oil. You don’t want to burn toxic chemicals into your cookware to leach out forever more. It’s a fairly expensive oil. I paid $17 for a 17 ounce bottle of cold-pressed, unrefined, organic flaxseed oil. As it says on the bottle, shake it before you use it.

Strip your pan down to the iron using the techniques I describe in my popover post. Heat the pan in a 200°F oven to be sure it’s bone dry and to open the pores of the iron a little. Then put it on a paper towel, pour a little flaxseed oil on it (don’t forget to shake the bottle), and rub the oil all over the pan with your hands, making sure to get into every nook and cranny. Your hands and the pan will be nice and oily.

Now rub it all off. Yup – all. All. Rub it off with paper towels or a cotton cloth until it looks like there is nothing left on the surface. There actually is oil left on the surface, it’s just very thin. The pan should look dry, not glistening with oil. Put the pan upside down in a cold oven. Most instructions say to put aluminum foil under it to catch any drips, but if your oil coating is as thin as it should be, there won’t be any drips.

Turn the oven to a baking temperature of 500°F (or as high as your oven goes – mine only goes to 450°F) and let the pan preheat with the oven. When it reaches temperature, set the timer for an hour. After an hour, turn off the oven but do not open the oven door. Let it cool off with the pan inside for two hours, at which point it’s cool enough to handle.

The pan will come out of the oven a little darker, but matte in texture – not the semi-gloss you’re aiming for. It needs more coats. In fact, it needs at least six coats. So again rub on the oil, wipe it off, put it in the cold oven, let it preheat, bake for an hour, and let it cool in the oven for two hours. The picture above was taken after six coats of seasoning. At that point it starts to develop a bit of a sheen and the pan is ready for use.

If you try this, you will be tempted to use a thicker coat of oil to speed up the process. Don’t do it. It just gets you an uneven surface – or worse, baked on drips. Been there, done that. You can’t speed up the process. If you try, you’ll mess up the pan and have to start over.

The reason for the very hot oven is to be sure the temperature is above the oil’s smoke point, and to maximally accelerate the release of free radicals. Unrefined flaxseed oil actually has the lowest smoke point of any oil (see this table). But the higher the temperature the more it will smoke, and that’s good for seasoning (though bad for eating – do not let oils smoke during cooking).

I mentioned earlier there’s a myth floating around that vegetable oils leave a sticky residue. If the pan comes out of the oven sticky, the cause is one of three things:

You put the oil on too thick. Your oven temperature was too low. Your baking time was too short.

It’s possible to use a suboptimal oil for seasoning, like Crisco or bacon drippings, and still end up with a usable pan. Many (most) people do this. But the seasoning will be relatively soft, not as nonstick, and will tend to wear off. If you want the hardest, slickest seasoning possible, use the right oil: flaxseed oil.

Your pan is certainly not ruined but it will take some work to restore it to it's former glory. The "rusty stuff" comment is a bit worrying though. It could be just overheated oils, but it could also be real rust. Rub it between your fingers - is it smooth and chunky, or more like sand? Let it dry completely then heat it with an open flame - does it smoke like oil?

Sandy and no smoke means real rust. Iron is an important part of your daily diet but usually not in the form of raw ferric oxide on your meat. Unfortunately the cleaning and seasoning methods listed elsewhere won't remove rust, and it is quite important that you get it all off.

Look in the local Yellow Pages under Sandblasting. There should be a couple of columns, both full serve and self-serve. 10 minutes in a sandblaster will remove the old oil, rust, anything else on there and leave you with a shiny pan with a pleasant fine-pebbled finish that is just perfect for cooking on.

Most of the sandblasting charges are for setup. If you go self-serve, take anything else that needs cleanup. Other cast-iron cookware, the barbecue grill, shovels etc.

There are many MYTH's about cast iron. Read this FIRST:
http://www.seriouseats.com/2014/11/the-truth-about-cast-iron.html .

My response: No big deal. In the restaurant we always had two cast iron pans on the back burners going white hot for the entire shift. 8 hours straight at times.

While/before it cools, throw some salt into it and toss it around a bit. This doesn't do much of anything other than lengthening the cool down process, absorb moisture, and help you scrape out any flakey stuff.

Your pan will be fine. Water/Moisture/lids introduce "rusty stuff processes." Did you add water or wipe it with a sponge or cover it while it cooled down?

After it has cooled and been cleaned, reheat to evaporate the moisture and then apply a thin sheen of oil.

It's not lengthy or difficult to recover from.

Edit: It doesn't mean you've burnt off your seasoning either, that diagnosis would take a few photos to determine.