How do you know the heart will restart are been stopped for bypass surgey. what if it didnt?

If no restart, death: The heart is stopped for this surgery using potassium and cold fluids. After bypass, the heart now has appropriate blood flow (the reason for the surgery). The heart is then rewarmed. It may restart on its own. If not, the surgeon may use special tools to shock the heart to regain function. This would be typical. If the heart is severely impaired going into surgery, recovery may be a problem.