The third floor of my parents' house has no running water. How do we get it to run again?

Frozen pipes can burst, It sounds like the water lines def. come up on a outside wall of the house, try to find them and blast them with a hair dryer, make a hole if necessary. That’s really your best bet, in case they do burst make sure you have access to the main shut off valve to turn the water off to the house. Has this problem happened before?

I agree with @TrenchantWit: get some direct heat applied to the blockage, pronto. The longer the pipes are allowed to remain frozen (and the colder it is outside), the more likely the blockage will become larger and eventually crack the pipe. It would be worthwhile to make a hole in the drywall to get that heat in. Start low, and the heat can work its way up. DO NOT apply flame. You can start a fire behind the drywall, and you may not even know it until it’s too late—you could lose the whole structure that way.

If it’s very cold outside and if the blockage has already persisted for some hours, then it would be wise to open up a larger hole in the drywall (probably where the vertical run of pipe comes up from below) and set an electric space heater a safe distance from the wall, but directed at the pipe.

Also as @TrenchantWit suggested, be ready to turn off the water to the whole dwelling if you do find a broken pipe. You’d have to be without running water for about 12 hours or so until you can get to a hardware store tomorrow and buy a shutoff for the third floor and fix that (and let the rest of the house have the water on), or call a plumber at weekend rates (still expensive, but at least it’s not Christmas night rate). With that in mind, you may want to store some buckets and other containers of fresh water in bathtubs for now… just in case.

I think I would go ahead and call a plumber. Surcharges notwithstanding, the damage a burst water pipe can do to your house and belongings can far outweigh the cost of a plumber on a holiday.

If the heat has been off to the third floor, and the pipes aren’t insulated and are running up an outside wall, it will take more than a few hours to have them thaw out. Make sure the faucet on the third floor are open, and that water is running to the second floor. Keep the heat up on the third floor, and open any cabinet doors under the bathroom sink. If there is a heat vent in the the third floor bathroom, shut the door to the bathroom, and let it get really warm in there. Do you have a space heater that you can put in there?

Definitely make sure the faucets are open, or you will have problems, and run the water on the 2nd floor too.

I have a bathroom over a crawl space, and fight frozen pipes every year. I have never had one break, though.

@PandoraBoxx It’s there now. Mom instructed me to place it there and point it toward the floor, as the pipes are right below the floor.

I “opened” (turned on) the faucets like you asked.

Mom also told me to turn the thermostat up to 85º (quite a bit of an increase on the bill though!) So with that, I’ll see what happens from there.