A hardier snake, but not an ignored one
A corn snake comes from temperate North America and handles cool spells that would trouble a tropical species, so a power cut is a monitoring job rather than an emergency scramble. That tolerance has limits: a corn snake in a house dropping toward freezing still needs help, and a snake that was digesting a meal when the heat failed needs watching. Begin by moving the enclosure to the warmest interior room, off the floor and away from windows, or insulating it in place if it is too large to carry safely.
Insulation and the thermometer habit
Wrap a corn snake's enclosure in blankets or foam board on the outside, leaving the ventilation path open, and put a battery thermometer where you can read it without lifting the lid. This species is an escapologist, and an outage is prime escape weather: lids get lifted in the dark, clips get left open after checks, and a snake that slips out into a cold house is far worse off than one that stayed put. Make one person responsible for opening the enclosure, and have them say out loud that the lid is clipped again after every check. A torch kept by the enclosure beats a phone light dropped into the substrate.
When a corn snake actually needs added heat
Judge added heat by the thermometer, not by anxiety: a corn snake sitting at ordinary cool room temperature for a day needs nothing but insulation and quiet. If the enclosure falls well below that and keeps dropping, place wrapped hand warmers under one end of the enclosure, outside it, so a mild warm zone exists with cooler space to retreat to. Never tape a warmer inside, never rest the enclosure on a people heating pad without a thermostat, and never warm the room with candles or a camping stove. Recheck after an hour; the goal is to slow the fall, not to recreate summer in one corner.
Feeding, water and the digesting snake
Skip feeding a corn snake for the whole outage and for a day after normal heat returns, because digestion is temperature-dependent and a cold meal risks regurgitation. If the snake ate within a day or so before the cut, keep it as warm as your measures allow, handle it not at all, and watch over the next several days for a regurgitated meal, which warrants extra care and a vet conversation if the snake then refuses food repeatedly. Check the water bowl at each visit: it should be full, clean and unfrozen, and this heavy-bodied bowl is also your quick visual check that the enclosure has not been disturbed.
Closing the outage out properly
When power returns, resist stacking every heat source back on at once inside a still-wrapped enclosure; unwrap gradually and watch the thermometer climb to the corn snake's normal range without overshooting. Inspect the snake once it is warm: normal tongue-flicking, smooth movement and clear breathing are the all-clear, while wheezing, mucus or a new lethargy that persists into the next warm day means a vet call. Note how cold the enclosure got and for how long, while you still remember. Putting the corn snake's outage instructions, escape warnings and check schedule into KinsLe Family Sharing means every family member, not just the usual keeper, knows exactly how this snake is looked after in the dark.