I
Totally agree keep calm! I lost power on Christmas eve for 36 hours where the inside temps of the house dropped to around 51 degrees. I had recently purchased a battery air pump from pondforse (Coop's dual pump wasnt available) and on intermittent setting ran both my 2 sponge filters for 40+ hours.
Things i could have done better;
1. Wrap the tank in a blanket earlier to keep it as warm as possible, I did it but not until the tank temps dropped around 10 degrees
2. Unplug the heater so when the power restores you can control how fast it warms the tank up. i was a few hours away when the power was restored and by the time i got home the tank was back to normal temps. Probably too fast to not stress the fish but thankfully i only lost 2 fish in my main tank. (I did lose all 9 in my QT tank but that was my fault, i didn't have a battery pump available for it and the fish were already weakened from medications.