Prepare something with unopened milk that sat outside the fridge?
What is "spoiled milk"?

"spoiled" milk is not a thing, that needs to be avoided, in most cases. As long as it doesn't have any mold growing or something, it most likely just started fermenting in the warm temperatures outside the fridge. However, it is perfectly possible that your milk didn't even start to ferment after 12 hours, depending on the room temperatures where you are. In case it has started fermenting it should smell sour and contain clumps. If it smells normal and had a normal consistency you can use it as normal. 

What to do with fermented/soured milk?

Milk that has started to ferment is called soured milk. It is used in many places in Europe, though most commonly in eastern Europe as far as i know, but probably in many places around the world, too. 

I'm from Germany and my mom always used sour milk wherever you would use buttermilk. It has a similar tanginess and texture, but of course contains a bit more fat. You could use it in cakes and pancakes, but also in brines or marinades, as well in a base for breadcrumb coatings or similar things. Speaking as a german, there are a variety of mostly forgotten regional dishes where soured milk is the main ingredient, mostly desserts and soups. 

Some people even drink it as is. I've heard, before lactose free milk was a thing, many lactose intolerant people prefered soured milk over normal milk, because of its lower lactose content due to fermentation.