Turn off the main water supply. Locate the main shutoff valve in the basement of the building and turn it to the "closed" position. Open a faucet nearby in the basement or first floor to drain excess water in the pipes.
Look at the front of the toilet shutoff valve and locate the small screw in the center of the valve.
Use the Phillips-head screwdriver to loosen the screw just a small amount.
Grasp the valve and turn it back and forth until it comes loose. You may need to loosen the screw some more.
Continue turning the valve back and forth until it moves freely. Retighten the screw and turn the valve all the way closed.
Turn the valve again in the opposite direction to open the water flow. Do not turn it all the way open; if you're able to turn it until it reaches its outer stop, slowly turn it back about a half rotation, as leaving it fully open could cause leakage and will make the valve more prone to sticking open.
Turn the main supply valve in the basement or elsewhere back to the open position. Flush the toilet and inspect the toilet shutoff valve. If water is dripping from the valve, turn the valve slightly tighter until the drip subsides.