Okay, this has probably happened before in your house, the sink stopper in your bathroom sink stops working. This is a repair you definitely can do yourself without any tools. Sometimes the sink stopper is broken at the bottom. If so, you will need a new one. They aren't expensive and you can get it at your home improvement store.
First, look under your sink. You will see something similar to the second photo. To get your new stopper in place you have to unscrew the cap where the stopper hardware attaches to the main drain pipe. Once it is unscrewed then put the new stopper in the sink. Then start screwing the cap back on making sure the metal rod going through the cap goes through the hole in the stopper. Voila! You're finished. Now the stopper should go up and down like it is supposed to. I just fixed mine today in less than 5 minutes.