It actually usually restarts on it's own. Sometimes a mild electric shock is used to restore normal rhythm, or the surgeon may massage the heart to get it going.

Defibrillation uses an electric shock to convert abnormal heart rhythms, called fibrillations, into a normal heartbeat (called a sinus rhythm). Defibrillation causes much of the heart muscle to depolarize (a change in electric charge within a cell). This depolarization causes the sinoatrial node (your heart's natural pacemaker) to reestablish sinus (normal) rhythm.

With cardiopulmonary bypass, you're actually weaned off the machine. Your body is re-warmed (they cool you down, which actually reduces risks of brain damage and other issues that arise from being put on the bypass), and your arterial blood gas will be measured. Arterial blood gas is a measurment of how much oxygen is in your bloodstream. It is measured via a blood sample from the arteries. Then, the anesthesiologist turns back on silenced alarms (they don't want to hear the flatline (no heartbeat) alarm throughout the surgery, so the monitor is put on "silence"), adjusts monitor and ventilator settings, and adjust oxygen flow rate, as necessary.

After the aorta is de-clamped, the heart usually starts to beat without help, but sometimes drugs (called inotropes- they are drugs that "tell" your heart muscles to contract; some inotropic drugs "tell" heart muscles to contract with more force; other inotropes "tell" heart muscles to contract less forcefully) are needed.

Cardiac (the medical term for anything to do with the heart) massage may also be needed to get the heart restarted. The medical personnel will use their hand and directly massage the heart itself. It's basically direct compressions, like in CPR, but instead of using the force from your arms and hands over the sternum (breastbone) to "massage" the heart back to beating, it's directly to the heart muscle itself.

It does sometimes beat irregularly at first, and an electric shock directly applied to the heart muscle may be needed to restore sinus rhythm, much like in automatic external defibrillation. Sometimes, the heart will fail to maintain a normal rhythm, and an internal pacemaker (a device that shocks the heart into sinus rhythm) may be implanted.