Whoops! I just noticed that your BATHTUB stopper has the problem, not the sink stopper. So, just ignore my previous "fix-it" post. You will have to tell us what kind of stopper handle you have on the tub before we can tell you how to fix it.
The stopper is controlled by a lever UNDER the sink which is attached to the plunger. It's easy to fix, crawl under the sink and at the BACK of the main pipe is the control and it is usually silver metal and comes out of your main pipe. Yours either came unattached or it is hung up on something. Just jiggle it around. If you have another sink in the house that is similar you can look under that one and see how it works and see how it is attached and then you can fix the problem.
Unscrew the front plate and make sure something inside is not jammed or broken. The fix is generally easy and if it can be dismantled, you can take it out and go to your local Home Depot. Those folks rock!
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