It's probably got a Thermostatic valve (TRV) there's a small brass pin under the control head that sometimes gets stuck down - in the fully off position....
http://davidkearns.co.uk/plumbingtips_files/trv.gif
Unscrew the collar around the lower part of the control head, lift it off and tap the pin or pull up on it with a pair of pliers - put the head back on.
If the radiator is warm at the bottom but not at the top then it will need to be bled - Bleeding lets any trapped air out - you will need a special radiator key (pennies) to do it. Just undo the screw a turn or so - do not take it out - hold a rag over the hole and when water comes out quickly screw it closed.
Good luck...
Peccavi, January 2010
