Nobody likes crawling around in a dirty crawlspace or basement but letís face it, plumbing comes into the house at ground level and snakes below the floor, and through walls so a bit of spelunking may be necessary. If your house has only a crawlspace below the floors, a pair of kneepads comes in handy. If you donít have a pair of Rollerblade kneepads that you are willing to sacrifice to the project, Walmart sells these at very reasonable prices and it will be money well spent.
Hopefully your plumbing exploration is not the result of a pipe that has already frozen. If you donít already know which is north and which is south in your house, invest in a cheap compass. Since even on a cold day the south side of your house gets more hours of sun (or at least warming light), an exposed north side is going to freeze first and thaw last. For the same reason, the temperature on that side will remain the most constant. Thatís great for shade loving plants in your summer or window garden but for the pipes under your houseÖnot so much. Regardless of direction, if it gets cold enough for long enough the pipes will freeze if they arenít protected in some way.
If you open a faucet during very cold weather and no water comes out, donít take any chances. Call a plumber. If a water pipe has burst, turn off the homeís water supply using the inside cut-off valve and then turn off the water main at the street. You may be able to quickly isolate the location by turning off an in-line valve to that area but be careful of doing this with water flowing from the source since pressure build up will worsen the problem instead of improve it. Emergencies like this make your initial investigation of the water system pay off very quickly.
It may be a good idea to call your water company to be sure that the problem is not a burst water main on their side of the system. If your neighbors have lost water flow also, it might be the water companyís problem and not yours.