Once a persons heart stops beating the person has about 6 minutes to get the heart started again before it leads to brain damage and possible brain death.
When your heart stops beating you stop getting oxygen and blood to your brain and without the oxygen and blood going to your brain and circulating through your body your brain cells begin to die.
If you don't get your heart started again within 7 to 10 minutes then you will usually die.
So it's very important to get medical attention for someone who has a heart attack and there heart has stopped.
You should immediately begin CPR and have someone else call 911 if possible but if not you should dial 911 and begin immediately doing CPR.
Keep doing CPR until the medical help arrives.
If possible get a defibrillator for home and get trained to use it because it could save a life.