This is probably because the valve is not shutting in the tank. As an emergency it is possible to apply some lubricant to the arm at the hinge after cleaning it so that it closes properly.
I get the feeling, from the tone of your question, that explaining what to do might be futile, i suggest calling a plumber and having the ball **** changed.
sound like your ball valve needs replacing, not a massive job if you have some diy knowledge and a large spanner, you can pick them up at diy stores quite cheap, and it just a case of unscrewing one and replacing it with the other.
there is usually a service valve on the supply pipe as well so you can just turn that off whilst you do the repair, so you won't need to turn all your water off.