i was playing some music(amp), switched off & couldn't start. battery was dead. a good samaritan took out his battery to give me a start but may have reversed polarity before correcting this mistake(did not see.. but suspect) the car started but cut off after a minute. Borrowed a battery and drove home(night time). by the time I was home the battery was dead but no batt/alternator light came on dash. What can be wrong?