Finding that the cold-water faucet doesn't have any water is frustrating, but having a problem with only temperature helps narrow down where the problem might be occurring. Water enters a home or building through one pipe from the water main. This pipe splits off at the water heater, with one pipe going to the hot-water heater and another continuing into the home to deliver cold water where it's needed. Networks of hot- and cold-water pipes continue around the home inside the walls. If only cold water is affected, that eliminates all the hot-water piping as a source of the trouble. The possible causes, however, can still be just as serious as those that affect any pipe, regardless of water temperature.
Check the Shut-Off Valve
Always check the shut-off valve under the sink if you live with other people or have had plumbing work done. If you live by yourself and haven't had anyone over, a shut-off valve probably wont be the reason the cold water stopped flowing, but if you have children, mischievous roommates or forgetful repair people, the lack of cold water could be due to something as simple as the shut-off valve. This valve is located under the sink; you'll see two knobs near the pipes leading to the hot- and cold-water lines. Try turning the cold-water knob and seeing if the water starts up again. The cold-water knob should be on the right, if you have a one-handled faucet. If more than one cold-water faucet is affected, though, the shut-off valve under each sink becomes less of a possible culprit.
Hot and cold water travel to your sink in different pipes, and in winter, it's possible that part of the cold-water pipe leading to your sink has frozen. After you've figured out which sinks have no cold water, you might be able to figure out where the frozen section of pipe is -- or at least the general area -- if you have plans showing the pipe layout for your home or apartment. If you don't have plans or can't figure out where the general area would be, contact a plumber or the maintenance department for your building immediately if you're in an apartment. In any case, open the taps so melting water doesn't remain in the pipe, and start your home's heater.
A more frustrating and difficult-to-fix cause could be a blockage in the pipe, such as from debris that made it inside from the water main, or corroded metal that's flaked off the inside of the pipe. If only cold water is affected, that would indicate the blockage is not outside, as the hot water remains unaffected.
Don't start tearing down your walls to get to possibly blocked pipes. You could misjudge where the blockage is and bust open the wall unnecessarily. When dealing with an unknown blockage in your water pipes, call a plumber to deal with finding and repairing the problem.