What happens if your head falls off is it possible to survive?

Entirely, throughout history many people have survived such shots. One such person was our president Lincoln. he survived for (some say) 3 hours. He was shot from point blank range with an 1847 model colt gentlemans pistol (single shot) to the cerebellum. So to answer your question, you could survive but if the bullet were to pierce certain parts of your brain such as your frontal lobe which controls memory, or your cerebellum which controls primal instincts and thing involuntary such as breathing you would be a vegetable.