So the filter wasn't moving water for awhile? The bacteria in it probably died, then when you got it running again all that dead bacteria caused an ammonia spike. Or the chlorine in the tap water killed off your bacteria (chlorine is in the water to kill bacteria.)
Most likely the water in the filter/sump went bad and the polluted the tank. I have read that even after a long power outage you should clean your filters before restarting. Sorry for your loss