Photoshop :: Sky Blown Out

I have taken a photo outside which has my subject and background is trees and sky. Sadly the sky has blown out and is practically white. What I wold like to do is replace it with blue sky and clouds. I have my original photo, then created a layer of the same image to create a more rich in colour by using overlay which is fine. But now I need to improve the sky, but when I create a new layer (blank) and click filer, render, clouds it overlays over the top completely.