If the sink has a push-type pop-up stopper, unwind it, remove debris and wash the stopper and plug hole with soapy water and an old toothbrush.
USE A PLUNGER if the blockage persists. Stuff the overflow opening with a wet cloth, position the plunger cup so it covers the drain hole, then fill the sink with enough water to cover the cup.
Plunge vigorously up and down until the debris is sucked out. Never use a plunger immediately after pouring chemicals down the drain.
Try a coathanger if the plunger didn’t work. Unwind and remove the pop-up stopper, bend a hook in a wire coathanger and feed it down the drain.
Fish around for whatever is blocking the pipe and try to pull it out.
Check the trap under the sink. Put a bucket under the U-bend and unscrew the couplings connecting it to the pipes. Let the water drain, then search for the blockage with coathanger wire.
Use an auger or plumber’s snake if you have one, as they have a longer reach than coathanger wire.
If you can’t get hold of an auger, call in an expert. To save time and money tell the plumber what measures you have already tried.