Unscrew and remove the thermostat head. In the centre of the valve still attached to the rad, you will see a small pin. This pin should move up and down when pressed. If it is not, it may be seized so tap it with a small hammer to try and free it.
Apart from this there is little you can do other than drain the system and fit a new valve.
It may be worth swapping the head with another working rad to see which bit is faulty. If it is a brand make, you may be able to replace just the head if it is faulty.
Hi, header says radiator thermostat? do you mean electrical room stat. or
a thermostatic rad valve?
If room stat. it's unusual for them to fail, normally whatever they are controlling fails to respond, depending upon system it could be a motorised valve, more info on system you have would be helpful, ie combi, 1 or 2 motorised valves etc.
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