Central heating won't work after taking radiator off for decorating!!!?

Hi, I took a radiator off the wall in the hallway this morning go I could decorate behind it. I hadn't tightened the inlet completely and lost about half a panfull of hot water from the system before i realised it wasn't coming from the radiator but the pipe. After i tightened the inlet the radiator came away no problems. After returning home after work my house is freezing because the boiler keeps cutting out as it seems there isn't enough water in the system. I have an indirect cylinder system which runs from a header tank. I ume the system automatically tops itself up from the header tank? The pump was making a funny noise (as though there wasn't any water) and when i bled the pump nothing came out. I've shut the whole system off now as all the radiators are cold. Maybe it needs to have the radiator back on to operate? Any ideas would be welcomed!