I lost power for over 10 hrs. Last time I looked at the temp it was at 75 . But I went to sleep and six hrs later it came on. I don't know how low it got. Whatcha think. Unplug it or go on..
you will probably still have some hatch. ive had temps drop to the mid 60s and still get over 50% hatches. if you open your incubator 15 minutes a day every day during the hatch, it also helps with resistance to power outage. if you don't, the hatch rate will drop but some will still come out. usually when the temperature drops like you are explaining, you will have more pullets hatch. supposedly cockerels are killed by the spike in temperature.
If your power ever goes out again here is what I did it could save your hatch and live chicks it did mine!
I had power go out and what I did was I filled a doubled ziplock bag with really hot tap water and laid it in the incubator to keep the temperature up. I was in the middle of chicks hatching day 22 so I also had to fill two gallon jugs with really hot water I lined the bottom of a box with angora wool from my rabbits (u can use rags or blanket cotton balls) anything to act as an insulator i then put a cloth on top of wool so their feet didnt get tangled in the fine stringy wool put one jug on each end chicks in the middle then covered the whole thing with a blanket. the temperature didnt start dropping for 3 hours then I just refilled with new hot water. an old hot water bottle would work great too. ( I couldnt find mine quick enough so I used what I had.