A toilet tank needs to fill up with water after each flush; otherwise, it cannot function correctly. Without enough water in the tank, the toilet will not flush with enough force to avoid clogs in the drainpipe. A toilet that does not fill up at all is useless.
Flapper valves are made of rubber and do not last forever. When the flapper valve wears out, it will not sit over the drain opening in the bottom of the tank securely, allowing water to leak down the drain. Replacing a worn flapper valve takes less than five minutes to do. Another possibility is that the chain on the toilet's flapper valve may have bound up, not allowing the flapper valve to sit over the drain opening. Jiggling the handle may unbind the chain, or you may have to remove the tank cover and unbind the chain by hand.
The water valve that comes out of the wall just below or behind the toilet controls the flow of water that fills the toilet tank. If the valve is closed, no water will flow into the tank, no matter how many times you press on the toilet's handle. You can open the water valve by turning its handle counterclockwise. Keep turning the valve counterclockwise until you can turn it no more, ensuring the valve is open completely.
If the toilet tank has some water in it but is not filling up to the water line inside the tank, the problem is with the float level in the tank. The float in older toilets sits on the end of a long metal rod. To adjust the float level, you must bend the arm upward. Getting the toilet to fill to the water level line may take some trial and error with older toilets. With newer toilets, you adjust the float level by turning the screw on the top of the float or by pinching a small clip as you slide the float up and down on its rod.
Clogged Fill Valve
The fill valve is the large cylinder-shaped piece inside the toilet tank that moves the water into the tank. Like faucet aerators, the fill valve may become clogged with hard water deposits or other debris. By shutting off the water to the toilet, removing the fill valve's cap and then turning the water back on for a few seconds, you can flush out the debris clogging the valve. If the fill valve still does not work, it may need a new washer in the cap, or the whole valve assembly needs to be replaced.