sime format c boiler-female in distress!!?

Hi, my heating has been off most of the summer and on swithcing the system back on it worked perfectly but i noticed lots of air in two radiators. I bled the first one and heat reached the top but on the second only a small amount of air came out but it was still cold at the top. I checked the pressure and it's gone to 0 bar, i tried filling it up by opening the water valve but the pressure remains at 0. What am i doing wrong and should the boiler be on or off when topping up the water pressure? thanks in advance.