A waste pipe carrying water from the sink etc does not go straight to the drains. It incorporates a trap in which water lodges to form a seal that stops drain smells from pervading your house. This device can also trap waste matter, which may result in a blockage that stops the water from draining out.
Periodic cleaning of the waste pipe helps to prevent blockages. Fit a wooden bung at the drain end of the pipe outside the house, and pour down a strong solution of soda and water. After an interval, pull out the bung and let the solution drain away.
If there is a blockage at the sink, try to clear it initially with a plunger. Old types of plunger consist of a rubber suction cup at the end of a short timber handle. Seal off any overflow outlet the sink may have by pushing an old rag into it. Next, place the mouth of the suction cup over the waste outlet. Rapidly push up and down On the handle to pump the suction cup. This should dislodge the blockage. Modern sink plungers look more like a bicycle pump, except that they have a cone-shaped end to fit into various sizes of outlet. Bung up the overflow outlet, position the device, raise its handle and bring it down with one sharp movement.
If the blockage is stubborn or you do not have a plunger, you must work from underneath the sink. There you will probably find a U-trap with a screw-in plug. Sometimes the plug has to be unscrewed with a spanner. But if it has little lugs, tap these gently with a piece of wood and a hammer to make the plug turn. If the trap has no plug, the whole section must be removed: it will be held by locking nuts that fasten it to the sink waste and the main outlet. A bottle trap ((bottle-shaped) has a cap covering the whole of its base. This is unscrewed with a large spanner.
Whatever the type of trap, place a bucket underneath before removing the plug or cap. Immediately water should gush out and with it, if you are fortunate, the material causing the blockage, If, however, the obstructive material is left behind, stick a length of wire (curtain wire is ideal) up the trap and wiggle it around until the blockage is cleared Replace the plug or cap, then run water from the tap to make sure both that the trouble has been cured and that the plug joint is waterproof.