If the pressure gauge on your system reads between 0-0.5 mbar you will have to fill the system. You need to find what’s called the “filling loop” (or “internal filling loop” if it’s inside the boiler). Both of these will have two taps or levers, which join a heating pipe to the cold mains pipe.
Open both taps/levers and you will hear water rushing into the heating system. While this is happening, look at the pressure gauge and watch the black needle rise to 1-1.5 mbar (if your gauge doesn’t have any numbers then it will sometimes have a green “OK” section). When the needle is in the correct place, turn off the taps/levers at the filling loop so that needle now stays in position.
Now your system will be fully pressurized and in some Vaillant combi boilers, start working again as they should do.
Hope this helps!