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.