It's common for a bathroom sink drain to become clogged. Many of the substances that go down the drain, such as soap, toothpaste and hair, can get fused together into a ball that keeps growing until it stops the flow of water. You can free most clogs with a plunger or by using a caustic drain cleaner to dissolve the mess. If neither of these works, it's a simple process to remove the P-trap and clean it out. At the very most, you may need a snake to persuade problem clogs further on down the drain line to move along.
Fill the sink about half-full of water and then use a plunger to free the clog. Stuff a rag into the overflow openings on the sink, then fit the plunger around the drain opening so that it makes a complete seal with the bottom of the sink. Pump it several times and then lift it off the drain and see if water runs out.
Let the water completely drain from the sink if it is running sluggishly but the drain is not completely blocked. Pour drain cleaner down the drain. Pour the amount specified on the container and let it work for the specified amount of time. Then flush the drain with water.
Use channel lock pliers to unscrew the clean-out nut on the bottom of the P-trap if the drain is still clogged. Put a bucket under the trap to catch the water before you remove the nut. Try to pull out any debris that gets stuck on the way out of the hole. If the sink has a pull-rod assembly, unscrew the lever from the drain tailpiece and check for obstructions.
Remove the entire P-trap if the drain is still stuck or if there is no clean-out nut. Unscrew the trap by hand from the drain tailpiece, or use channel lock pliers to unscrew it. Then swivel it away from the sink and unscrew it from the wall stub-out. Pour out the water, and then clean the drain by spraying a jet of water through it with a garden hose. Re-attach it to the wall stub-out and the drain tailpiece and tighten the nuts.
Clean obstructions further down the drain line by inserting the end of a plumber's snake into the wall stub-out. Unravel the snake and push it in until you feel resistance. Then unravel about 18 more inches, set the lock on the snake and crank the handle clockwise while you push forward, driving the snake further into the drain. Repeat until the blockage is cleared.
If the drain runs sluggishly and you can find no signs of blockage, the vent may be clogged. Check the vent opening on the roof or the side of the house for obstructions.
Drain cleaner is highly caustic. If you suspect there is any in the P-trap when you remove it, wear rubber gloves. If you get any in your eyes, flush them out with water immediately.