Depending on the type of battery, the complete drain might have done it in, or it may need a more powerful charge than can be given by your tender. Take it to a Sears Auto Center - they have deep cycle chargers there, plan to leave it overnight. Make sure they don't just throw it on the quick charger (30-60 minutes). If it comes back bad, you know it's the battery. Sometimes if you drain a battery completely, it'll take more than a tender can manage.