There are several possible reasons why your car won't turn on when you put the key in the ignition. It could be that your battery is dead or your alternator isn't working properly. In some cases, a fuse may have blown, preventing the lights from coming on. Another possibility is that a ground wire has become disconnected or broken. Finally, if all of these things check out okay, it's possible that there is an issue with the starter motor or solenoid. If nothing else works, it's time to call for professional help.