If there are no fish currently in that tank now, I would add water that is near enough to the required temp as poss (why make your thermostat work overtime having to heat up the whole tank?) and set your thermostat to what you would normally have it set for those fish.
Also, as an added caution, if you have another filter set up, I would use some of the mature filter media from that to add to the filter in this tank because where the temp as been so low for that long you could find that your filter bacteria have died off too, not just the fish. I'm not entirely sure at what lowest temp the bacteria can survive at.
Then when you come to add your surviving fish back to their original tank, float them in their bags for several hours, like you would normally when introducing new fish, to acclimatise them to the water temp (the temp of the tank water will slowly warm up the water that is in the bag with the fish until both are the same).
Once you release them back into their tank, leave the tank lights off for several hours to help reduce stress.
Really sorry to hear of all your probs - I had a very similar incident not too long ago myself and it's quite upsetting to lose fish that you have treasured and cared for. In my case the heater broke but I didn't notice it until the water temp had dropped out of the "safe zone" on the thermometer.
Good luck with it all!
Athena
