If you come upon an unconscious victim who is not breathing, what should you do?

In an unconscious or semiconscious victim, especially one positioned on his or her back, the most common airway obstruction is the tongue. The tongue — which is a muscle — may relax and block the airway.
A victim with a suspected airway obstruction must be checked immediately for breathing and, if necessary, the airway must be opened.
The head-tilt chin-lift is the most reliable method of opening the airway.