To state the bl******g obvious, you have lost the pressure in the system, hence the boiler won't fire up or at least it tries and senses no pressure in the system and shuts down. You need to re-pressurize the system. This is done by letting mains cold water into the system whilst monitoring the pressure guage to ensure you don't over pressurize the system. To let mains cold water into the central heating system. This usually involves opening two valves which are normally linked by braided flexible pipe. (I have a Vailliat and thats the arrangement on mine) You'll probably end up laying on your back to see the two valves that you need to open. Open them slowly, you should hear the mains water rushing into the system. Once the pressure is in the normal zone don't forget to turn the two valves to the off position.
Hope this helps.
Richard, November 2008
