Hopefully any fireworks were minimal. If you don't have a phone or a flashlight, then before hooking up any cables to either car, flip on the jumper car's headlights and lay out your jumper cables so when you turn the lights back off everything is already lined up where it needs to go. And always double check.
Damage can range from only a dead battery to fried components (starter solenoid, fuses, control modules, etc). It will take some troubleshooting on your part. Maybe get the battery tested and (likely) replace it, then hook it up and go from there checking things. May need to get alternator tested as well depending on what you get when you hook up the new battery and try starting the car.
Hopefully you got lucky and damage is minimal.