When temperatures are consistently at or below freezing:

• If you have pipes that are vulnerable to freezing, allow a small trickle of water to run overnight to keep pipes from freezing. The cost of the extra water is low compared to the cost to repair a broken pipe.
• Open cabinet doors to expose pipes to warmer room temperatures to help keep them from freezing.

If your pipes freeze:

• Shut off the water immediately. Don't attempt to thaw frozen pipes unless the water is shut off. Freezing can often cause unseen cracks in pipes or joints.
• Apply heat to the frozen pipe by warming the air around it, or by applying heat directly to a pipe. You can use a hair dryer, space heater or hot water. Be sure not to leave space heaters unattended, and avoid the use of kerosene heaters or open flames.
• Once the pipes have thawed, turn the water back on slowly and check for cracks and leaks. PennLive

Click HERE for more great information on dealing with frozen pipes, from PAWC.