hoe can I get the pressure up! Glow worm 24cxi boiler?

Pressure had dropped.Reason unknown (not used much during the summer) and pressure dropped below 0.7 bar (should be 1.0 bar). Red light came on and boiler cut out. However, turning the filling loop knob has no effect at all. According to the instructions it's as straightforward as turning it gently until pressure rises appropriately - does this mean the valve is broken?