Boiler pressure at 0 even after turning the pressure valve.

[jo82]

I have a Sprint Rapide 90nf combi boiler and the pressure gauge has fallen to 0. I dont know when but the hot water was fine a few hrs ago. I just switched the CH on and noticed it was'nt working so checked the pressure. I tried to increase the pressure via the filling loop but the gauge does'nt move. I can hear the usual hissing sound when i have done it before but no luck. I checked for leaks under the boiler but it seems fine.
Is the problem to do with the boiler rather then the radiators if the pressure wont increase when i do it manually?

 

[bengasman]

Either a big leak, or the gauge and sensor are blocked.

 

[JohnD]

look outside where the pressure relieve valve has a pipe going through the wall from the boiler.