Reviving someone who has just died???

Okay, this may be somewhat of a weird question, but it's always been something I have not completely understood and questioned. When someone overdoses on prescription medication, such sedatives, they are putting themselves in a self induced coma right? There heart muscle becomes so relaxed it stops beating? BUT there brain is still alive for so long after? Why is it...say when celebrities overdose, like Anna Nicole Smith or Heath Ledger for example, why can't they be revived? Emergency workers have mobile electric heart shockers with them, are they being used? Even if they are used, are there instances when it just doesn't work? Our bodies are electrical systems, there has to be a way to get that heart beating again! OR is it that they ARE able to get the heart going...but the person is brain dead, therefore can not breathe on there own or have any other vital signs, therefore is still dead even if the heart is beating? I'm confused please explain!