If the control valve is a thermostatic type they sometimes suffer from the control pin sticking in the closed position. If yours is a TRV take off the plastic top and gently pull up on the central brass pin with a pair of pliers to release it - then reasssemble.
If you don't have a TRV make sure that the inlet and outlet valve on the other side (lock shield valve) are both open (anticlockwise).
If the radiator still remains cold consider removing it and washing out any sludge that may have built up inside.
Peccavi
February 2009
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