To unblock a sink give the following methods a try before calling a professional!

 - Boiling water to melt grease - pour boiling water down the plughole a little at a time to allow it to work its way through the blockage. 

Give it some time to melt any fat or soap build up and then flush through with hot tap water.

 - Salt and boiling water for mild blockages - get as much water out of the sink as possible so that the solution can get to the  blockage. Pour around 75g table salt into the plughole & pour boiling water over the top of it.

Leave for a few minutes to allow the solution to reach any grease, fat or food debris & then flush with hot tap water for around 30 seconds to wash away the solution & debris.

 - Use a plunger to try to loosen or move the blockage - utilise any water already in the sink (or add some warm    water if it is empty) which will help create an airtight seal for the plunger.

Place the plunger over the plughole covering the plughole completely - cover the overflow hole with your hand or block with a cloth. Once all outlets are covered push the plunger up and down until you can feel the resistance- do this several times until you can hear or feel any movement in the plughole, you should hear a gurgle or see the water draining out.

Run the hot tap for around 30 seconds. If there is still a blockage repeat a couple of times.

 - Baking soda and vinegar - get as much water out of the sink as possible so that the solution can get to the       blockage. Pour around 150g of bicarbonate of soda into the plughole and then around 250ml white vinegar over the top, covering with the plug or a damp cloth. 

Once it starts fizzing the solution has started to work so leave for around 15 minutes to allow it to work its way through the blockage and then run the hot tap to clear the pipe.

 - Baking soda and salt for overnight clearing - get as much water out of the sink as possible so that the solution can get to the  blockage. Mix around 150g of bicarbonate soda & 150g table salt together and pour down the plughole (use a funnel if you have one). 

Pour into the plughole & leave overnight if possible, but at least 3 hours if not. Pour boiling water down the plughole to clear the blockage residue away.

 - Drain unblocker tool / rod or wire coat hanger - anything long and thin with a bend or hook on the end would do for this, preferably something flexible. You need a hook or a bend at the end in order to dislodge the debris and pull it up the plughole.

Feed your implement down the plughole until you feel the blockage, move the implement up and down to try and either break it up, dislodge it, or hook it and bring it back up the plughole. If you don't get it all the first time, do it again until the waste pipe is unblocked.

Flush any remaining debris away with hot tap water.

 - Wet/dry vacuum for stubborn blockages - if the blockage is still not moving and you have access to a wet/dry      vacuum, put the hose directly over the plughole & run for around 30 seconds. You will need to seal around the edges of the hose with a cloth. Flush with hot tap water and if it runs freely then the blockage is gone.

Only ever use a wet/dry vacuum, DO NOT use a normal household one. 

 - Liquid or gel sink unblocker for really stubborn blockages - these are designed to dissolve blockages without   damaging your pipes or sink (check they are suitable for your sink material), but only use the recommended amount and cover the surrounding areas to avoid splashes.

Follow the instructions to the letter as these can be dangerous chemicals and then flush withe pleanty of hot tap    water to dilute the solution and flush as much of it away as possible.

Always wear rubber gloves for protection & cover any surrounding areas in case of splashes or spills.