If you are trained in CPR (Cardiopulmonary Resuscitation):
Check for pulse and respirations. If both of these are absent, proceed with CPR as you were trained.
If respirations are absent, but there is a pulse, proceed with rescue breathing as you were trained.
If you are not trained in CPR (Cardiopulmonary Resuscitation):
Call emergency services.
If the person is unconscious, begin to push hard and fast on the center of the chest.