Saunier Duval has gone off!! Please Help!!!!?

Hi,Iv got a Sauner duval Boiler F23 its about 5 years old.
The fans fine,pumps fine,control pcs's are fine.
But i can't put any water into the boiler.
Ive took all the water out of the radlators but the pressure gauge is still on 1.
on the left hand side inside the boiler there a switch, if you press it the boiler starts to work and its nexts to a small black pin that i think is jamed,but i can not put any water into it.
is my pressure gauge gone?
i have also put in { Sentinel x200 descales }from the top radlator.
But still i cant put any water into it from the loop under the boiler.
What can i do now????