Blocked sink?
Water not running away normally?
What can you do before calling an expert?

First, try using a plunger to clear the blockage, you may have to block the overflow to prevent any pressure from simply escaping there. If this doesn’t work, it may mean that the pipe below the sink is blocked.
Put the plug into the plug hole and place a bucket under the pipe. Unscrew the pipe carefully ensuring that the bucket is correctly positioned below to catch any water that escapes. Rinse the pipe out thoroughly, removing anything that may be blocking it, typically fats, hair or foodstuffs depending on the location of the sink will create an obstruction over time if not kept clear. If it seems clear, then the blockage may be in the waste pipe, use a piece of stiff wire to poke it clear. Fix the pipe back below the sink let the water run away. Pour boiling water down and washing up liquid down the plug to ensure that it is fully clear.
If these procedures have not resulted in successfully solving the problem, it is important that you consult an expert. They will be able to ascertain the cause of the problem and find a safe solution without risking damage to the system.