Look up online any advice on how to unclog a drain, and you'll find at least a few articles admonishing you for considering drain cleaner. These liquid drain openers are in almost every store, so the advice to avoid them may seem strange, if you've never had to deal with unclogging a sink. However, there are some good reasons why you shouldn't use them including that the cleaner might not work. It helps to understand what could cause such a popular product to not do what it's supposed to.
The Clog Is Made of Non-Dissolvable Material
Liquid drain cleaners work by both dissolving the clog and by making the walls of the pipe slippery so that the clog starts to move. However, if the clog is made of a material that won't dissolve, or that the particular chemicals in the drain cleaner won't affect, then you'll just end up with a sink full of drain cleaner. Sometimes the clog is actually wedged between the pipe walls, such as when a toy gets stuffed down a drain by a bored child. (While most sink drains have a plunger with a cap blocking the opening, some drains don't.)
If the cleaner can't dissolve the material, and if the material is wedged so tightly that nothing will make it drop down the pipe on its own (i.e., it won't move without physical force, as opposed to letting it slide down after coating the inside of the pipe with drain cleaner), then you need another method to move the clog.
The Clog Is Actually a Tree Root
Sometimes the clog is not in your drain, but farther down the pipe where it runs under the yard toward the city sewer system. Tree roots can get into pipes through small cracks and then grow into the pipe. You would have noticed a slowdown on drains throughout your house if this were happening, although the final clog can happen quickly.
The Cleaner Isn't Strong Enough
If the clog is particularly big, such as a huge clump of hair and gunk that finally closed up the drain, the cleaner might not be strong enough to dissolve the whole thing. So, part of the clog might be gone, but enough is left over to keep the drain plugged up.
Before your sink drain reaches this point, however, have it cleaned out. Hire a drain cleaning service like American Minuteman Sewer & Drain to get each drain as clean and open as possible so you can avoid last-minute messes.