Can anyone help? We have a Vaillant termo compact boiler - about 3 years old. Yesterday we noticed that we had no hot water. The central heating seems to work fine, but not the hot water. The boiler is connected to a megaflow water tank. Basically, the boiler has gone very noisy and the pressure reading on the boiler has fallen to zero bar (I think it should be about 1.5). We turned the boiler off and then on again and it started up for a while (very noisily) but then stopped again. We think it maybe that there isn't enough water in the system - if it is this then is it easy to refill the system?? Any help would be greatly appreciated!

Your Vaillant will not operate unless you have a system pressure of 1.5 bar on the gauge.
You have a leak in the system somewhere (possibly) which has caused the boiler to shut down. Refer to your Customers notes on how to repressurize, fairly straightforward procedure through a valve on the filling loop.
If your not confident then you have no other choice but to call in a Plumber or a person with the know-how.
Mel.