How can I get the pressure back up on my combi boiler?

I've removed a radiator to decorate. The pressure on my valliant combi boiler has gone down to zero I'm presuming that the reason the water pressure is down is because I can't fill the pipes back up with the radiator off. Do I need to bleed the remaining radiators or do I need to get some sort of device to close off the pipes on the removed radiator?. I've checked for leaks and tightened all the pipes on the remaining radiators. Apologies for any lack of clarity. Suggestions welcomed