Because a toilet water supply valve isn't normally turned off very often, it can become stuck open. Hard water deposits typically build up around the stem of the valve and harden, according to HGTVPro. This can be an annoyance if you need to repair the toilet and can't shut the water off. A stuck toilet water supply valve can be a real problem if your toilet develops a leak and you need to shut the water off quickly. Before that happens, you need to loosen the valve by using an adjustable wrench.
Place a rag underneath the toilet water supply valve to catch any water drips that may occur during these steps.
Locate the bonnet nut on the valve. This nut is located at the bottom of the shaft the valve handle is attached to. The shaft comes up and out of the middle of the nut.
Use an adjustable wrench to loosen the bonnet nut a just a quarter turn or so. Try to turn the valve. If you can't, turn the bonnet nut another quarter turn. Keep loosening the nut until a little at a time until you can turn the toilet water supply valve.
Re-tighten the bonnet nut on the supply valve if it leaks at all. Turn the valve off and on a few times to break up the hard water deposits.
Turn the toilet water supply valve off, then back on, once or twice a year to prevent it from becoming stuck, according to HGTVPro.
Never try to force a toilet water supply valve to open with your hand or with a tool. You can damage the valve, or break it and cause a water flood.