Mineral deposits can make these water turn-off valves difficult to operate especially if you have hard water. Or the shut-off valve may just have been tightened too tight. If the Tiolet water shut-off valve is stuck and you can't turn it in either direction try the following.
NOTE: You may want to first turn off the water supply line to your house before attempting these tips (in case something goes wrong). Also make sure you are turning the shut-off valve "Clockwise" or to your right, in order to turn the water off / close the shut off valve.
1) Get a crescent wrench and loosen the nut that's just in back of the shut-off valve knob. This should loosen the valve enough that you'll be able to turn the shut-off valve (off or on).
2) Spray some lubricant like WD-40 around the shut off valve.
3) Lightly tap the shut off valve with a hammer.
4) Turn the shut-off valve "Clockwise" to turn the water off, then turn the valve to "Counter-Clockwise" to loosen up the valve.