Some people have their t-stats and heat sources hooked up to computer UPSs. I don't think I've heard any conclusive evidence of how well this works. Also, apparently some t-stats say in the instructions that you should not do this.

You can keep various types of heat packs on hand for use in an emergency.

If you have a gas stove or gas hot water heater, you can fill bottles with warm water. Even if you have electric, the water in the hot water heater will stay warm for a while after the power goes off, so it could extend the length of time your snake stays warm at least somewhat, as long as the warm water doesn't get used for something else first.

You can take your snake out of its enclosure and keep it warm against your body.

You can move your snake into an area that you are able to keep warm, such as near a fireplace. I'd be very careful about it getting too smoky though, as that might be as likely to cause an RI or other problem as a chill would.

You can take your snake with you to a friend's house or a hotel.

You can take your snake out to the car and start the car up off and on as necessary to keep the interior warm enough.

Obviously, some of these methods are better than others, but if you get caught without power for an extended time, any source of heat might be better than none...