Biggest concern depends on your setup. Average stocking with a free standing tank you'll have to deal with heat. Heavy stocking you will have to deal with O2 depletion. I had the power go out for 8 hours about a week ago, thankfully my fishroom was stocked light. The house dropped to 60F, 1k gallons in a 8'x16' room dropped to 76F. I've had it go out before when I was stocked pretty heavy, after a few hours they were having issues. Quick solution was to start pouring water with a huge cup, they started looking better. Next step is fire up the generator.
Since I was stocked light last week I started eating when the power went out, ham sandwiches by candlelight. If you aren't stocked heavy raid the fridge, the fish will be fine, can't have good food go to waste!