Stop tap has no effect on water flow
It's possible that the valve has failed or that something is blocking it.  At this point, your best bet would probably be to turn off the water upstream and remove the valve.  Watch for anything that might have come loose from somewhere and clogged it up.  You should also be able to shine a flashlight in one end and see the light looking in the other.  When the valve is open, you see some light, and closing the valve darkens the side opposite the flashlight.  If moving the valve has no effect, you would likely benefit from replacing it.