To thaw frozen water lines that haven't burst, shut off the water supply valve and heat the frozen pipe length with a hair dryer, space heater or heat lamp. Alternatively, wrap the pipe with temperature-controlled heat tape.
Shut off the water supply valve to the frozen section of the plumbing to help prevent water from leaking out of the pipes due to pressure. Use an incandescent or infrared heat lamp to thaw the pipe, placing a cookie sheet on the other side to radiate the heat back if the pipe is close to a wall. Use a high-power hair dryer for safe, effective thawing, or direct a small space heater toward the pipe for stronger heat. To use heat tape, wrap the ribbon around the affected section, and plug it into an outlet to start thawing. Never use propane torches to thaw pipes, as they create a fire risk.
For a frozen water line behind a wall, place an infrared lamp in front of the wall to heat the pipe through it, or remove the wall portion to access it. Another option is to turn up the home's heat and wait for the pipe to thaw naturally. To prevent future freezing, wrap the problem pipe section with heat tape or insulation.