The heart will automatically try and start, provided that it has oxygen, energy, correct pH and the correct electrolytes.

In surgery, the heart has its blood supply replaced with ice cold potassium chloride solution. The potassium is completely the wrong electrolyte, and the temperature is low, so the heart stops.

To restart it, the blood supply is reconnected and once warm, fresh blood with oxygen and the correct electrolytes reaches the heart it will restart. It may restart erratically on ventricular fibrillation, so it may need a shock to get back into normal rhythm.

The reason during resuscitation you can't usually restart a stopped heart is because you have to reverse the cause of the heart stopping first. Often it is severe illness, with major biochemical abnormalities or multiple organ failure, which can't easily be reversed.