Drain cleaners are all fine and good, but beware if you have cast iron pipes. Cleaners will eat a hole in your pipe and you will have an even larger problem. Flush the toilet next to the sink, if it flushes normally, then you know the clog is near the sing and not farther down the line where the toilet ties into the line. A snake is the best thing to use, just take off the P-trap under the sink and inspect it for debris. If none is found, run the snake into the drain coming out of the wall. A blowbag on the end of a garden hose would be a good follow up, which you can purchase at home-depot. It expands in the pipe and basically pressure washes the line.
