If the tank is over stocked the bacteria in the filter probably died while the power was down.
The tank being over stocked has nothing to do with the bacteria dieing in an hour. Thats not enough time for it to die, and bacteria doesnt die from tanks being over stocked.
My guess is ammonia shot threw the roof in that hour without filtration. While the airation would insure gas exchange at the surface, it does nothing for ammonia building up in the tank.
Tank turned back on, and the filtration could not convert the ammonia fast enough. Too much ammonia in the tank for too long.
The tank would have to be incredibly over stocked, and under filtered for that to happen though.
Horrible results, but a good learning lesson.
He was a stunning ray. Good luck with your new venture.