Does the radiator have a thermostatic valve, if so see if you can remove it without breaking it - the heat makes the plastic on some valves brittle, once removed tap not hit, the pin and see if it pops up, also check that the lock shield valve at the other end hasn't been turned off by other members in the house.
I have a job of sheltered housing flats in Cambridge, the quinn barlo valves fitted to the 300 odd rads there seize regularly, so leave the trv fully open once freed up to avoid it seizing again.
b penny, February 2009
