I have a 98 hyundai tiburon. recently the battery light went on when I was driving. the next day my girlfriend left the headlights on and the battery died. We tried to jump it with no luck. I got the battery charged and tested and it tested ok. I put it back in the car and still not luck. The interior lights come one and it dings when the key is in the ignition and the door is open. the hazard lights work, but the headlights, dash lights, and radio do not. There is no noise when I turn the key and I noticed the cables were a little loose. Could a loose connection be the issue?
If the battery is fine, then you may have a fuse issue, which headlights not turning on at all would be a strong indication of (those are incandescent/halogen types, so they put out some kind of light down to scary low voltage and amperage). Check fuses for the components not working and replace any blown ones with the proper rated fuse, which is usually indicated on the fuse box, if not in the owner's manual or service manual. Never replace fuses based only on the ratings of the blown fuse.
There are several reasons why a car would not even attempt to start. A few to try first...
Flat battery - you have tested this, so we can assume it's ok.
Loose/poor connection to the battery - this would result in no power to anything, so I'm guessing this isn't the problem as you have some power
Immobiliser - If you have one fitted, this might have got upset when the power was lost, and isn't de-immobilising to allow you to start the car.
Loose/poor connection elsewhere in the charging/starting system - this would be more likely to result in intermittent starting rather than none at all.
Check all the fuses as Paul suggests, although I don't think these will be causing your main problem. If the car has an immobiliser, that might need resetting.
It turns out I had 2 fuses that were burnt out. Some ignition one and the alternator fuse under the hood. I'm hoping they were blown when the car was initially jumped and there is no other underlying cause. Thank you for your responses they were very helpful.