Clogs in sinks and tubs often develop over time. In a best-case scenario, you periodically clean out any hair or other materials from the drains to avoid a clog. You can also pour boiling water down drains to clear them out.
But if your drain is completely stopped, you can try using a plunger. To create the proper vacuum and seal, you will need to seal off the overflow drain and add some water to the tub or sink basin. Do not plunge a drain line after chemicals have been used. Also, don't use a plumber's snake on a tub drain.