Yes, a heart that has completely stopped beating cannot be restarted by defibrillation. It only works in a condition called ventricular fibrillation, when the muscle fibers of the heart are still contracting, but are no longer “in sync”. This causes the heart to twitch chaotically, which is not an effective way to pump blood. And without blood pumping, the heart itself does not receive any oxygen as well, so it will eventually go into a complete flatline after a few minutes of untreated ventricular fibrillation.

The electric shock helps the fibers resynchronize. If you want to see the effect directly, here’s a video:

https://www.youtube.com/watch?v=HCbawp9ZSnY

Be warned, it shows an open chest and an exposed heart, most likely during heart surgery. They are using spoon-shaped internal defibrillation paddles. In the beginning, you can see the heart in ventricular fibrillation. It’s twitching chaotically and not pumping any blood. After defibrillation, it starts contracting rhythmically again.