pressure loss worcester bosch

Hi my Worcester bosch pressure is at zero and I followed the instructions to raise the pressure, but nothing happens and the pressure stays at zero. What else can I do?

My central heating system is based on a Worcester Greenstar system boiler. The system has been losing pressure slowly for the past year or so, needing a top up through the filling valve every fortnight or so. In the past week, the pressure loss has become much more severe, with the pressure...