a puff of smoke and now nothing 69 valiant electrical issue

so my son was taking off the dash (he had taken the ground wire off the battery, as told) and he was replacing light bulbs that were burned out on the instrument cluster. (we have only had the car for a month, its a project). He reassembled, put ground wire back on and it started to smoke near the fuse box (i think, i wasn't here and he was in front of car( He quickly removed ground wire) now, nothing. No lights, car dead. I checked and the battery is fine. Used a light meter and not one fuse lights up the meter. Two questions:
1. what do you think happened?
2. How do i fix this? is it a simple (swap the fuse box), or am i looking at a new wiring harness?