Is it possible for your eyes to fall back into your head?

Not at all. The eyes sit in the eye socket (orbit), which has a cone shape formed by the skull bones. The tip of the cone points back toward the brain and has a hole that is much smaller than the eyeball. Also, the eye is tethered in place by muscles, so it cannot fall into the head. Trauma to the face can break the bones in the floor of the orbit and the eye can sink down a little, but it cannot fall back.