plumbing
The basin cold water tap in my bathroom over the last week has gone from fully flowing, to half pressure, to a dribble, then to a drip and now nothing comes out.
The 3 other taps in the bathroom work correctly, including the cold water tap on the bath.
Are there a series of simple things to check before calling out a plumber?
Per Tester101's answer a bit more information
There are two main types of valves - washer and cartridge. In the washer type, the stem of the valve (the part topped by the handle) moves up and down on a thread. It has a washer at the bottom that, when closed, presses agaisnt the seat of the valve stopping the water. The higher the stem raises the washer, the more water that flows.
In a cartridge type faucet, the stem twists without rising. The water flows through holes in the cartidge and the body of the valve. The more the holes are aligned, the more water flows.
It sounds like you have a washer type. The tension on the stem, which controls how easy it is to turn, is usually controlled by a large nutlike cap that fits over most of the stem. The thin shaft of the stem comes through a hole in the center of this cap. The cap can usually be adjusted by tightening and this should make the stem slightly harder to turn. It should also prevent the stem from turning on its own due to water pressure. Tighten the cap by turning it clockwise a very small amount.
Occasionally water temperature can change as the washer heats up or cools down. This is usually found on much older washers and faucets, but can often be fixed by opening the stem and changing to a modern washer.
The pipes are not likely to be blocked. The problem will most probalby be in the tap. Most taps usually have a aerator before the exit and that could be blocked up from dirt.
To clean the tap there is a tool to remove the aerator but it is not necessary. You can use pliers or a the correct size key.
There is also a helpful video here
And this is what the part looks like when its a bit clogged.
With the clog that you mentioned it must be pretty bad- So when you remove the aerator you will most likely see allot of dirt follow it. Open the taps for a minute to get it out of the system.
This dirt usaully comes from the main water supply so installing a filter for your main supply for the house if a good option to stop this nuisance. There also options to improve the drinking qualities. Ask you local plumber depot for advice.
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Best Answer
I could be that the connection between the handle and the actual valve is bad - either it's loose, or stripped.
Is it possible to take the handle off (I'm assuming you have a separate hot & cold handle from the question) to check the condition? Once it's off, you should be able to use a pair of plyers to try to open the valve directly.
If that doesn't work, check the shut-off valve under the sink - if as @aphoria mentions it's something blocking the line, that's another place to check. Turn it off, then back on again, which may dislodge the blockage.