One of your headlights could be broken for a variety of reasons. A common cause is a blown fuse. There's also the possibility that the headlight bulb has burned out. If you recently hit a pothole or were in an accident, the headlight housing may have been damaged. If you don't know what's wrong with your car, take it to a mechanic or dealership so they can diagnose the problem. They'll be able to figure out what's wrong and fix it quickly. In the meantime, driving with only one working headlight is illegal in most states. So, if your car only has one working headlight, fix it as soon as possible!