Hyundai Repair: starting, jumper cables, slow blow

Question
my son left his lights on and now the car won't start. tried jumping myself and AAA nothing. charged battery overnight and still nothing can't even get the horn to beep. it's a 1995 elantra which we only use as a station car so I really don't want to sink a lot of money into finding out it's to expensive to fix.

any ideas would be appreciated.

Answer
Check the slow blow fuses in the box attached to the positive battery cable.  If the jumper cables or charger were hooked up backward, the main fuse is probably blown.

You can check the battery with a voltmeter.  It should read about 12.5-12.8V.  Try again when attempting to honk the horn or operate something else electrical on the vehicle.  If the battery voltage stays up, but the devices don't work, you should check fuses.  If the battery voltage goes very low, then the battery isn't fully charged or won't accept a charge.