Usually when you open a bathtub's drain, the water will drain out over the period of a few minutes. When the water sits at a standstill, you need to determine the cause of the lack of draining and address the problem. If you are not able to resolve the problem yourself, contact a plumber for professional help.
Built-in tub stoppers over time will come out of adjustment, causing them to either allow water to escape through the tub's drain when the stopper is supposed to be closed, or the stopper will sit too low in the drain and not allow the water to drain out of the tub. Grab the edges of the stopper and pull it and the linkage assembly attached to it out of the drain. If the stopper was not lifting up enough, the water will immediately begin to drain out. Turn the nut on the underside of the stopper so the stopper sits higher in the drain.
Bathtub drains develop clogs over long periods of time. Hair, soap residue and other debris washes down the drain, but not all of it flows out of the drain pipe. Instead, some of the debris sticks to the walls of the pipe, accumulating slowly until the flow of waste water is constricted. If you pay attention to how quickly the water drains from the bathtub, you will be able to catch clogs as they are forming, instead of waiting until the drain is completely stopped up and you are left with dirty standing water you cannot drain. Try reaching into the drain opening either with your fingers or a pair of needle nose pliers and pull out any debris you can grab.
Plunge the Drain
Once you know the bathtub's drain has a clog, use a plunger to apply pressure on the pipe, dislodging the clog from its place. Wet a rag down and shove it into the tub's overflow drain opening. Coat the edges of the plunger's cup with petroleum jelly, helping the cup form a tighter seal with the bottom of the bathtub. Place the cup over the drain opening, then push and pull the handle hard at least 12 times.
Snake the Drain
Use a drain snake or auger to remove tougher clogs from the bathtub's drain. Pull the wet rag out of the overflow's opening, then use a screwdriver to remove the screws that hold the drain's cover in place. Pull off the cover along with the tub stopper's linkage that is attached to the cover. Feed the auger into the overflow drain's opening as you turn the crank on the handle clockwise. Once you have pushed the plunger beyond the P-trap underneath the tub, turn the auger's crank counterclockwise and pull it back out of the drain. Replace everything you removed from the tub, then run the hot water for five minutes to flush out any debris leftovers.