Defibrillators are often mistaken for being responsible for reviving a person. but, what can revive a person after they have died or "flatlined" ?

CPR: Defibrillators will revive a cardiac arrest due to a disorganized rhythm called ventricular fibrillation but not in the absence of any rhythm or as you call it flat line which is the other mechanism of cardiac arrest. The latter is often treated with drugs such as epinephrine or a temporary pacemaker. But the initial management of both consists of chest compressions(CPR) until definite treatmen.