Another possibility is that the radiator has been isolated at the valves either side of the rad.
One side may have a cap with a ribbed surface, if so, just try turning it (by hand) anti-clockwise. At the other side of the rad there may be just a smooth covering cap. Take off the plastic cover and you'll see a flat protruberance (!) which you can slip an adjustable spanner on. Try to turn it anti-clockwise (gently). If it moves, open it fully until it stops, once again taking care to do it gently.