The Washer is jammed or there is a service valve (stop cock) somewhere on the pipe leading to the tank.
Look for a valve in line first
If no valve then CAREFULLY but quite forceful,- rapidly move the ball cock up and down - if this works then the washer was stuck and may need changing.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix plumbing.