If the blockage is not too serious, you may be able to clear it with baking soda and vinegar, which foams up to loosen the clog. Run the water afterward. A commercial liquid drain clog remover can be used instead of baking soda and vinegar. Also, try using a plunger to unclog the drain. If you still can't clear the blockage, then you should try using a plumber's snake. Remove the drain cover, or the overflow panel, to gain access for the snake. Push the snake down until you hit the clog, then push and pull the snake vigorously - or turn it on if it's electric - until the blockage is cleared. Replace the drain cover afterwards and run the water to rinse away any remainders of the clog.