There have been several of these today, Car. First thing to try is the TRV (thermo red valve). Give it a tap with a mallet. The pin is possibly stuck closed
In fact .. unscrew the retaining ring that holds the whole TRV onto the valve. Some are a clamp. The TRV body can be removed .. there is no water connection here.
Look for a small pin protruding from the TRV valve body on the radiator .. in the centre. It should be up under spring pressure. If stuck down, try and grip it and wriggle gently so it pops up.
This sort of thing happens after period of non operation.