The Insurance Institute for Business and Home Safety (IBHS) reports that freezing pipes is the number one source of cold-weather property damage and estimates that the related water damage can end up costing a homeowner $9,000 or more. If you've just woken up or arrived back home to discover a burst pipe somewhere in your house, it's too late to prevent a leak, of course, but the sooner you respond the better. The first step is to shut off the home's water supply at the main shutoff valve. After that, you have a couple of options, both of which eventually involve a plumber.
Step 1: Turning Off the Water Supply
If you don't know where you main water shutoff is, it's a darned good idea to familiarize yourself a.s.a.p. If you live in a cold-winter climate, your shutoff valve is most likely located in your basement or crawlspace, because the water service line to the house enters the building from underground, where it won't freeze in winter. Look for a pipe coming through the foundation wall or up from the ground or floor. This may or may not have a meter attached to it. The shutoff valve is located after the meter. The valve may have a round handle (like the kind on outdoor hose spigots), or it could have a lever or other type of handle. If it's round, turn it clockwise to shut off the water. For other types, move the handle all the way to the marked "OFF" position. By the way, when you turn the water back on, make sure to open the valve all the way; it's meant to be completely open or completely closed.
If you can't find the shut off valve in the basement or crawlspace, follow the main water line (typically a 3/4" pipe inside the house) toward your hot water heater; sometimes the shutoff is near the water heater. Houses in warm climates may have main shutoff valves outdoors.
Quick Question: Should I Shut Off the Electricity?
Many sources recommend shutting off the electrical power to the entire house or at least to the area of the leak, to prevent fireworks resulting from the unhappy marriage of water and electricity. You do this by switching off the circuit breakers in your service panel (breaker box). Flip individual breakers to the "OFF" position for individual circuits, or flip the main breaker (usually at the top center of the breaker panel) to shut off the power to the entire house. Frankly, I'm not comfortable with either endorsing or discouraging this practice, but I am certain that you shouldn’t go near the breaker box if there's water anywhere in the area, including on the ground or floor below and on your person (that means you, in TV cop language).
Step 2, Option A: Call a Plumber
After shutting off the water supply, you can cut right to the chase and call a plumber to have the burst pipe replaced. Hopefully it's during regular business hours; if not you'll just pay more for the "emergency" service. A plumber can also check other pipes and devices that may be prone to freezing to make sure they're ok and can tell you how to prevent the problem from happening again.
Step 2, Option B: Patch the Pipe, Then Call a Plumber
If you really want to get the water going again or you can't get a plumber right away, you can try to repair the pipe TEMPORARILY with a patch kit. (If you didn't happen to notice the "temporarily" in all caps in the last sentence, please go back and read it again. A patch is not a permanent pipe repair.) You can buy these at home centers and hardware stores. The quickest type to use includes a rubber patch, to wrap around the pipe, and two plates that screw together to clamp the down on the patch. In a pinch, you can wrap an inner tube or a piece or neoprene around the break and clamp it down with a few house clamps.
Note: You have to drain and thaw the pipe before applying the patch. Do this by opening the faucet or other fixture supplied by the pipe, then carefully thawing all frozen areas of the pipe with a hot water bottle or a hair dryer (again, don't mix water and electricity), working from the fixture end toward the frozen area. It's possible that the pipe burst between the frozen section and the fixture.
Dealing with Water Damage
Whether you do it yourself or call a pro, don't wait to start drying things out. Mold can start to set in within 24 hours. Dealing with a flooded area is a subject too big for this paragraph, but here's the crib sheet version: If you're confident you can get to all of the water and wet surfaces yourself, great; just do it as soon as possible. But if the water damage is extensive and/or seems to have entered areas you can't see, like inside walls and under floors, call your insurance agent. Professional water damage mitigation needs to start right away, and it can be eye wateringly expensive.