First of all, stop freaking out !!! I've seen mama hens get up and leave their eggs for a whole day in weather with temperatures lower than that and they hatched just fine. Of course in an incubator it's more difficult to have great hatches to begin with, but if it was only for one day and the temp was 75 the next time you saw it, I'd say plug it back in and let it heat back up to where it was before you left. Just as it takes some time for an incubator to get to a certain temp., it takes just as much time (sometimes longer) for it to cool down, so you may not have had a temperature drop as bad as you think. Also, don't just crank the heat back up to get the temp higher fast because spikes in temp are just as bad, if not worse, than drops. So just let the incubator warm back up like you normally would and once it's again at the set temp., you can candle the eggs again. But I'd wait at least two days before doing so and even then, don't toss any that don't look viable because some can just be slower to develop now that the temp dropped some. Hope that helps and good luck
-kim-
