Time and elements can cause water valves to become stuck. Whether you are installing a new faucet or doing repairs on sprinklers, being able to turn the water valve on and off is critical to being able to do the job. If pure strength is getting you nowhere, you will want to apply a lubricant to the frozen valve penetrating grime, dirt and corrosion. This should be enough to let you free the valve. Once it is freed up, you may want to replace the valve to prevent future problems.
Coat the top and bottom of the valve with the lubricant. Apply the lubricant generously, trying to get it into the cracks and crevices.
Allow the lubricant to soak on the valve for ten to fifteen minutes.
Place the wrench around the valve and try twisting it clockwise to open and counterclockwise to close. (Note that you may need a second wrench or pliers to crisscross it over this wrench if you do not have enough leverage over the valve.)
If you are unsure what lubricant to use, ask a customer service representative at your local home improvement store for suggestions. You are looking for something that works quickly and will dissolve dirt, rust and grime.