Dead from bumper to bumper

My 76 NYB recently lost ALL electrical.
I changed the flashers for the hazards and the turn signals, when I put everything back together and tested them with a loose connection at the battery the flashers worked fine.
The terminal connection at the battery was still loose when I tried to start her up, I heard a snapping sound, and she went dead.
I thought I had blown the fusible link at the battery and replaced it ....... still dead.
Thoughts and suggestions are appreciated.