If you are talking about the bathroom sink, check to see if the toilet flap is working. Since they both drain from the same pipe, if the flap is stuck (to open), that will let the odor come back up to the bathroom.
.Caustic Soda, this will clear the pipe in the wash basin and get rid of the smell
Try dumping a 1/2 cup to a cup of baking soda down your sink followed by vinegar, it will start bubbling and cleaning grease and smells from your sink.
Put in the stopper and pour one cup bleach in the sink. Slowly (to avoid splashing) fill the sink with water and let it go out the overflow hole. This is often a place that bacteria build up. This will also clean the drain and trap since thats where the water goes from the overflow.
Did you clean out the p-trap? Sometimes hair and gunk get stuck and smell when they degrade.
