In your home, under every sink and behind every toilet where there's running water, there's a simple valve that controls the flow of water to the faucet or tank-refiller. This valve is called an angle-stop. Paying occasional attention to these devices is a good way to prevent the necessity of spending many hours (and/or dollars) replacing them when they cease to function properly, and it also becomes necessary to repair or replace the fixtures the water supply of which they control.
Especially if they water in your system is "hard"--rich with minerals, salts, etc.--it is a good idea, every so often, to simply twist the valve from the open position to closed, and then open them again.
Unfortunately, if you have left this go for a while, when you endeavor to turn the stem of the valve, it may be frozen open--likely due to mineral build-up inside the valve. This will be evident if, when you try to turn the handle, it is stuck, or resists hand-loosening or tightening.
If this is the case, REMOVE the screw holding the 'handle' to the valve-stem, remove the handle itself (which is cheap-o, pot-metal at best, and too fragile to withstand the torque of a wrench), and turn the stem with a pair of channel-lock/slip-joint pliers.
Operate the valve frequently enough that it seems easy to turn. Do not lubricate. (In many, especially older homes, the valve is soldered ("sweated") to the supply line; in newer appliances, the valves are threaded.)
Do this for every angle-stop in your house, twice yearly (I do it when the clocks change).
Remember: "Righty = Tighty; Lefty = Loosy" for all threaded equipment except that which regulates gas-flow, which is always the reverse...