Understand what the cut actually takes away
A power cut is a triple loss for a reptile: heat goes first and matters most, but lighting and any powered humidity equipment stop in the same instant, and the enclosure begins drifting toward room temperature on every front at once. How urgent that is depends on the species in the glass. A hardy snake or a leopard gecko in a mild house has hours of grace; a small tropical lizard in a cold house has much less, so your first move is to think about which animal you keep, not to reach for generic panic. The encouraging truth is that reptiles are built for temperature swings in a way mammals are not: a slow cool-down over an evening is an inconvenience to most healthy animals, not an emergency.
Hold the warmth in and leave the animal in peace
Close the enclosure and keep it closed: every opening dumps warm air and invites handling the animal does not need. Cover the vivarium with a blanket or towels for insulation, taking care that fabric cannot sag onto lamp fittings when the power returns. Put off feeding entirely until heat has been back for a good while, because digestion needs warmth the animal currently does not have, and skip misting too, since evaporating water steals heat. Check conditions with a thermometer that does not need mains power rather than guessing from how the room feels to you; you are warm-blooded and a poor judge of a reptile's comfort. If the enclosure truly starts to chill, a hand warmer wrapped in cloth at one end offers a gentle warm spot the animal can choose or avoid.
Watch for the few situations that need action
Most power cuts need patience, but a few need a plan: a cut stretching into a long, cold night; a house that is itself becoming bitter; a very young, elderly or unwell animal; or a fish-room-style setup where pumps and filters matter as much as heat. In those cases the answer is relocation, not improvisation: the reptile travels in a secure, insulated tub with a wrapped warmer to a home, vet practice or boarding facility that has power. Do not try to heat the enclosure with candles, stovetops or portable fuel heaters in the room; open flames and fumes around a sealed glass box create a worse emergency than the one you started with.
Do the return-of-power checks before you relax
When the lights come back, the incident is not over until you have checked the equipment: confirm the thermostat is reading sensibly and its probe has not been knocked loose, that lamps and heaters have actually restarted rather than tripped out, and that temperatures climb back to the normal range steadily rather than overshooting. Offer water once the enclosure is warm, and wait for a normal activity pattern before the next feed. Instead of trawling forums in the dark during an outage, put a follow-up question to KinsLe's Kimmy; she answers using the KinsLe Answers library.