||06-25-2011 02:03 PM
As Kok328 said, something's wrong with the valve and you'll need to shut off the water and disassemble the valve to see what went wrong. I am providing you a picture of the inside of these toilet shut off valves so you can know what to expect. Most likely, the washer has become dislodged and will need to be removed or the entire shutoff may need to be replaced.
If you open the valve and find nothing wrong with it then it is also possible that the toilet fill-valve has gotten plugged up with sediment/washer materials and will need to be cleaned/cleared of sediment. Chek out a video on how to clean/clear right here: