How do I raise water pressure on my Greenstar 25si/ 30si?

I have a Greenstar 25si/ 30si combi boiler. About an hour ago it made an unpleasant noise and I turned it off at the wall. Now realise there is no pressure and the burner isn't lighting when I reset. I have tried to add water pressure. Using the white key and turning the valve (only turns clockwise not anti-clock) water starts seeping out rather than filling the system. I think I need to know how to do this. Hope once the system is full the burner will light.