system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is it that makes a heart able to be restarted in some cases and not in others?
gpt
It's a common misconception, but if you're think about a defibrillator with the shock paddles, those don't restart your heart.  If your heart is not beating, a defibrillator will do nothing.  

Defibrillators work when your heart isn't beating the way it should be, typically this means that the heart is barely pumping at all.  Instead of being a normal, strong heartbeat it's basically nothing more than a twitch or a flutter.  It's pumping, but not well.  This is because of the electric signal controlling the pumping isn't correct, so the strong shock attempts to reset the tempo.

When your heart isn't beating at all, there is no electric signal telling it to beat, so shocking it will do nothing.