How to unblock a sink or basin
Blockages are caused by food and other waste getting stuck in the u-bend or pipes. A simple plunger is the best way of clearing this. Don't use strong chemicals, especially caustic soda. If they don't work you will have a sink full of nasty liquid to deal with.
Remove as much of the water as you can from the basin by bailing it out with a cup. Seal the overflow by holding a cloth over it. Place the plunger over the plug hole and pump up and down quickly. This pushes and pulls the blockage apart. This should clear it.
If your sink is regularly getting blocked it could be that fat waste is causing it. Read more here.