No. The membrane surrounding the eye, the conjunctiva, prevents objects from going behind the eye. If you think you lost a contact in your eye, it would be very irritating and you should be able to grasp it by looking all the way up or all the way down.
Impossible. There is no 'opening' for the lens to slip behind the eye. It will either be hidden under the upper or lower lid in a little cyl-de-sac known as the fornix.
No. The surface of the eye is covered by the conjunctiva. This surface forms a barrier and prevents things like contact lenses from getting behind the eye.
No. A soft contact lens can roll up and go under the eyelid, but cannot get lost behind the eye. You should see your eye doctor to remove it if your eye is irritated. Most of the time the lens ends up lost somewhere (on the floor, in the sink, stuck to your clothing, etc.) while you were trying to remove it, and it is not really still in the eye. Your doctor will look under magnification.
No. The front of the eye is sealed from the back by a continuous apron of conjunctival lining. A contact lense or anything else for that matter cannot penetrate behind unless there is a traumatic defect in that lining. Contact lenses including rigid lenses are too soft to cause such trauma.
Not likely. There is a tube of skin called the conjunctiva that extends from the edge of the iris, goes halfway back along the surface of the eyeball, and then turns around forward to join the inside of the eyelids. Unless you tear through this skin (which would bleed and hurt a lot), there is no way for a contact to go more than half way back on the eyeball.