Radiator not working

One of our radiators isn't working and I can't figure out why. A bit of background, as much as I know:
We moved in in March. We noticed that the heater wasn't working and asked the previous owner. He said that he had taken the radiator off to wallpaper behind it and had turned off both valves in order to do so. After putting the radiator back on he turned both valves on again. This does seem to be the case - I've checked and they do both seem to be open (as well as the thermostat being on max). However, none of the pipes going in or out are hot.
I've tried bleeding the radiator, and (cold) water comes out straight away, so there doesn't seem to be an airlock in there.
I've tried turning off all the other radiators in the house except for that one and turning the heating system on (something someone recommended to me) and that hasn't worked either.
Any tips on anything else I can try before calling out a plumber?
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