If it's a bathroom sink, then you can open the pipes by unscrewing them to remove a wad of hair/a clog. Put a bucket underneath to catch the foul water. If it is the kitchen sink, then a plumber should be called, especially if you have a garbage disposal.
Before using harsh chemicals on that clogged drain try peroxide. It is reasonably priced and found at all pharmacies and grocery stores. Just pour it down the drain and listen for it's bubbling sound, then rinse with cold water. I do this once a week and no more problems.
Pour a half gallon of bleach down your drain, and allow to sit overnight without being used. Follow the next morning with boiling water. Follow up by using a drain snake to remove any leftover residue and debris. If the drain is still running slow, repeat the process the following night.
First thing to try would be putting some "Liquid Draino" down the drain. I that does not work you might try and us a long "snake" which is a long metal wire that you can wind down into the drain to try and loosen the clog.