just add some drain cleaner containing "lye" . once you get the water moving, pour some boiling water down the drain to dissolve any grease. you could also use plain ol bleach to dissolve any hair that might be contributing to the problem. a drain maintenance procedure would be to add some baking soda(about 1/2 cup) followed by some boiling water,(about a gallon or so), use this once the problem has been resolved. worse case you will have to use a pair of plumb pliers to remove the trap under the sink, use a pail to catch any water (there will be some, guaranteed)....do this as a last resort