Before you use those nasty chemicals! A blocked trap is your most likely culprit. And, since plumbers are lazy

this can be very easy to fix, even for a rarely-do-it-myselfer.
If it is a sink drain, and you have PVC (plastic instead of steel or whatever those old ones are made out of), and you can access the "trap" (the S curve in the pipe below the sink), you may have an easy (although a tad gross) way to fix your problem. But if you already poured chemicals down the drain, you'll need to take special precautions to avoid getting burned by the chemicals in the trap.
How traps work: Sewage lines generate nasty methane gas, plus plenty of stink - neither of which you want in your house. So plumbers install an S pipe, called a trap. It holds water as a barrier between the sink and the rest of the sewage system, so none of that nasty gas gets into your house. It's also a great place for a clog to hang out.
Assuming you are chemical free and meet the rest of the criteria get a bucket that fits under the trap. Bail as much water out of the drain as possible. Then unscrew the trap, should be finger tight (or looser, some plumbers helpers are *really* lazy

). Look into the trap and say "Yeeeech!". Then clean it all out, reassemble and test.
Be aware that when you unscrew the trap, all the water still in the drain above the trap will come rushing down. Hopefully, you'll get most in the bucket, but you'll probably want some rags handy to wipe up. Let us know how it goes.