What happens if the sky falls?

This is like asking what would happen if the ocean fell. The sky is basically an ocean of air. It can't fall because it's already on the ground. You are surrounded by air right now. As you go up higher, the air gets thinner and thinner until it's gone. When you look up and see blue, that's just the color of lots and lots of air. If you look at something on the ground that's far away, it will be tinted blue too.