1: is it full? bleed til water out
2: does it have a thermostatic rad valve? does it work if you remove plastic head from valve, generally collar nut to slacken to remove.
3: older system? could have dropped off system as some do. turn all others off and leave cold one open. set heating away and hopefully it will start to work. then re-open others not fully open, but enough to work. water will flow easiest route.
4: might be faulty rad valve not allowing circulation
depends on age and system set up really but that should give you a idea of poss problem
....., March 2009