If your cream puffs are flat, much like they collapsed in the oven, it's possible that you opened the oven door too soon while they were baking. It's imperative to let the pâte à choux set properly and begin to brown before you attempt to open the oven door, otherwise they will collapse from the loss of heat. If this happens, you can't fix them. They will still taste good, but the shape will be more like a thick pancake. Another reason could be that there's too much egg in your pâte à choux. Too much egg will lead to a softer pâte à choux that doesn't hold its shape well.