It sounds like the valve has had enough. Turn the water off at the incomming supply. If the ball-cock assembly is the old brass type with a screw cap on the end facing the ball, take the cap off and remove the bits noting the order. Take these bits to the local plumber or DIY store and smile nicely! Failling this, remove the whole assembly and take it to the shop. You will need two pairs of adjustable spanners and some white plumbers tape (PTFE) for re-assembly. Go-on, you can do it.
Carl
July 2005