One cold radiator

Took a radiator off in an upstairs bedroom to do some decorating. Working fine before I took it off. Put back on, bled it, but it is stone cold and has been for three days. The inlet pipe seems cold, but the oultet pipe is warm about a foot away from the valve. definately no air in the radiator. Is the obvious answer that the outlet valve is broken/blocked up. If the latter, how do I clear it. Wife driving me up the wall asking if the damned thing is working yet. Any hints gratefully received.

If it's a thermostatic valve, take the head off and push up and down at the pin which sticks up. Even hammer a bit. But don't pull it out or you'll get wet. If manual valves, heavy treatment can make them stick/jam/break shut. Only way to be sure is to drain rad and see if each valve does let water out.

Thanks for your help. They are manual valves. If I have to replace one, is this difficult to do or can a novice like me manage it?