Sounds like the (water) pressure has gone out of your system. There should be a 'filling loop' that allows you to connect to the mains water temporarily and let water into the system. This thing looks like a flexible braided hose - usually with a couple of stopcocks at each end. Simple open the stopcocks (you'll hear water moving into the system) - watch the water pressure guage rise on your boiler - turn it off as the needle moves into the 'red' area (usually no more than about 3 bar at most) Close off the stopcocks (strictly speaking the braided hose should also be disconnected, but they are often left on.
You'll probably find you'll have to bleed your radiators and maybe repeat this operation until the CH system is completely full with no air pockets.
Bear in mind that the pressure shouldn't go down very often - if it does it a lot you have a problem (possibly a minor leak at a valve or connection)
Graeme, October 2005
