How to fix my Biasi M90F.24s?

The pressure in the boiler went down when we bleeded one of the radiators. The worse came when while we were looking for the top-up valve we flicked the pressure release valve above the motor, all the water came draining outside. The D.H.W is still working but it seems the radiators are all emptied. How do we put water back into the Central Heating and raise the pressure to normal?
Many thanks.