A stuck bathroom sink flange is a result of plumber's putty hardening between the flange and the sink. The flange is part of the sink drain basket. Installation of the sink basket involves placing plumber's putty under the flange to prevent leaks between the sink and the flange. Over time, the putty will harden under the flange making removal difficult.
Other times, a stuck flange is not stuck at all. Some people mistakenly overlook the retaining nut under the sink. A bathroom flange that is stuck will require some force to break the putty seal.
Look under the sink to ensure that the retaining nut is off the sink drain basket. Remove the securing nut from the sink drain basket with a pair of channel lock pliers. If the sink drain spins while you are removing the nut, you will need to secure the sink basket.
Ask a friend to hold the handles of a pair of pliers inside the sink drain basket. This will keep the sink drain basket still while you are removing the retaining nut. If necessary, spray some penetrating oil on the threads and allow the oil to sit for one hour. Go back after the hour and remove the nut from the sink drain basket.
Cut around the perimeter of the sink flange between the flange and the sink with a utility knife, if the sink flange is still stuck to the sink. This will score some of the plumbers putty under the flange.
Go under the sink and hold a small board against the bottom of the sink drain basket. Tap the board with a hammer just enough to break the seal of the plumbers putty.
Ask your helper to slide a putty knife between the flange and the sink while you tap on the wood. Have your helper work the putty knife around the sink flange until the flange releases from the sink.
Do not tap the board hard with the hammer. This could cause the ceramic or porcelain sink to break. Steady light tapping will break the plumbers putty free.