If someone's heart stops from ventricular fibrillation, will a defibrillator restore heart rhythm or is it too late?

That is : The main utility of a defibrillator, and it usually works.

ICD: Icds are implanted to monitor the electrical activity of very abnormal hearts to see if a life threatening rhythm occurs. If it detects one of these rhythms, it will charge its capacitor and fire a shock to defibrillate the heart back to stable rhythm. In appropriate patients these devices save lives.