Rust or debris caught between the flapper valve and the bottom of the tank is the most frequent cause of a toilet tank that does not fill. A faulty valve or seal is also common. This problem is usually accompanied by the continuous and noisy flow of water from the tank to the bowl.
If there is something blocking the flapper valve from closing fully, or the valve or seal are damaged, the toilet continually flushes, causing the tank to empty at the same rate it fills; in this case, the bowl's drain is normally still closed, leading to the bowl overflowing until the incoming water is shut off. A handle that is stuck open also causes the tank to continually drain, although this is generally easier to fix. If there is water on the floor of the bathroom, the cause may be a cracked toilet tank that requires replacement; if the mechanism between the float ball and the fill valve is broken or poorly adjusted, the tank may stop filling prematurely; if there is something wrong with the tank's water supply, such as a blockage or a frozen pipe, the tank either fills very slowly or not at all.