My phone all of a sudden has service but I havent paid my bill in weeks. what does this mean?

This means that the phone company has made a mistake and accidentally given you phone service. If you an honest person (which I'm sure you are) you would use your free phone service to phone them and tell them about the situation. If you are like most people you would not phone them and, one day when you urgently needed to phone someone, you wouldn't be able to because they would have realized their mistake and quit your phone service. So you can either pay for your service and make that urgent phone call, or you can not tell them and not make that urgent phone call, but I'm sure you'll be honest - it's you choice.