It really depends on the ambient temp in your home and how long it lasts.
We found that most of our tanks don't start showing major heat loss until about the seccond /third hour.
I have has fish loss from the 6+ outs. My house is normally in the 60's WITH heat.
One morning there were tanks in the 50's. Unfortuneately, we keep tropicals. Not just semi-tropicals..full blown I- want- it- just- like- your- bathwater tropicals...
Fish are hardy despite us..I lost very few considering.
It is worth getting some heat packs form auto zone or a camping store or the like.
At least if you are awake and home you can ziplock them and toss em in to keep the tank temp stable longer. Or stick your fish in a smaller tub of water if you expect it to be long(like a pole down) and keep them warmer longer that way.
Either way if there is no air and it get real cold, you will lose your biological media and prolly a few fish.
"Fish! Thank You! Oh, tropical huh? Did you buy a heater? Auuugh!"
Mega-pet stores prolly should not be allowed to sell animals as retail "items".