If your car won't start, it could just be a dead battery from leaving something on, like the domelight or headlights. But the battery could be dead from a faulty alternator not keeping the battery charged as it should. Assuming the alternator is working and nothing was left on, batteries do die. First look at the sticker on the side an see when it was manufactured. If it is 5 to 6 years plan on it dying soon. To see exactly how much pep it has left, take it to an auto parts store, and ask them to perform a load test on it. That will tell you how much reserve capacity it has and if it is failing. And when buying a battery never buy the cheapest, get a mid grade or top grade.