Although there could be extenuating circumstances such as roadwork, broken pipes or more serious issues that will cause the water in your sink, tub or shower to backup, the normal and everyday cause is simply hair clogged in the drain.
Whenever we bath, wash our face or even brush our teeth, the release of the water down the drain will wash any hair located in the sink, tub or shower down the drain. Eventually, this hair will accumulate and cause the water to collect in the sink, tub or shower.
If you are like my wife, this accumulation of hair, soap and skin cells is a very disgusting concoction of debris that she actually gags when she attempts to clean it. That is the reason that husbands, boyfriends or other less squeamish exist in the household. So the following suggestions apply to only the less squeamish of us out there.
1.) If possible, remove the screen or filter that covers the drain. In sinks that have a pop up type of water stop associated with stopping the water from flowing down the sink, this will need to be disconnected. Some pop ups will simply unscrew from the top of the stopper, others will need to be disconnected below the sink. However, in all cases, you will have much better luck if you can remove the screen or filter and the stopper from the top of the drain. In most tubs and showers, the screen over the drain is either snapped down into the drain pipe, or it is attached with a small screw in the center of the filter.
2.) Once you have easy access to the interior of the drain by the removal of the screen, filter and pop up stopper, you will be able to normally reach the hair and other debris that is clogging the drain. I use a wire coat hanger with a bent hook at one end to slide down the drain and hopefully pull up the wads of debris. You will be amazed at the amount of hair and accumulated matter you will normally remove. Caution, there is normally a smell to this operation, this is normal and will go away, once the trap is refilled with water. The trap is the piece of piping that looks like a shepards staff or U shaped piece of pipe. This piece of pipe is called a trap due to the fact that water will remain in the bottom of the U and will trap any sewer gases from exiting through the drain and into the atmosphere. When you start to remove hair and debris, this effective seal could be drained allowing the sewer gases to escape. This is the smell along with the smell of the accumulated hair and other debris lodged in the drain.
3.) If the simple act of removing the screen and filter as well as the insertion of your wire coat hanger does not appear to be unclogging the drain, then I would first suggest the use of a plunger that is designed to force water under pressure down the drain. This forcing of water down the drain will in many cases, cause the hair and debris to dislodge and rinse down the drain.
If the use of a plunger does not work, then the use of a long snake that is inserted into the drain and pushed down until the debris clogging the drain is cleared. There are several different types of sink snakes that are designed to unclog your sink, tub or shower drains. Some of these snakes are designed to be spun, so that the head of the snake rotates within the drain, further improving the ability to clean out the drain.
4.) If we still have water remaining in the sink, then the removal of the drain piping below the sink is probably necessary. This will not work with the normal tub or shower, due to the lack of access to the bottom of the drain. In these cases, there is normally a cleanout that has been installed in the sanitary line that will allow you to more deeply and thoroughly snake the drain. In most sink installations, the removal and disassembly of the drain piping under the sink is an easy task, simply unscrew the connections. However, you will need to protect the bottom of the sink from flooding water with the placement of a dishpan or shallow pan to catch any water that remains in the drain.
5.) The use of a cup of baking soda poured down the drain and then a cup of vinegar is claimed to dissolve hair and other debris from drains that cannot be mechanically cleaned. This solution should be left in the drain for up to 5 minutes to allow the baking soda and vinegar to work its magic. Good luck with this one, I would prefer to physically remove the hair and debris than try to dissolve this blockage, however, if you cannot access the hair and debris, you should try this option.
6.) There are numerous other types of drain additives that can be poured down the drain in an attempt to dislodge the blockage. However, please make sure that whatever you are pouring down your drain is compatible with your piping as well as your sewer system. If you have a septic tank, the addition of a chemical into the system, may cause a negative reaction and kill all of the bacteria that you are counting on to break down the sewage that is being deposited into the system. Read the instructions and use caution with some of these chemicals, due to their tendency to be somewhat caustic and dangerous if used incorrectly.
7.) The last option is to hire a professional company to come and snake the drain. In most instances, this will guarantee that the drain is cleared, however, this will cost money and will not guarantee that the drain is not either broken or blocked by something that cannot be cleared. For example there could be tree roots that have grown into the sanitary line exiting into the street or the septic system, or the line could be crushed. All of these possibilities exist if the more common methods of clearing the drain are not successful.
Good luck, but I bet it is only some hair and soap debris.