You can try taking off the thermostatic head and carefully raising the pin manually, but usually the valve jumper gets stuck to the valve seat and often the pin rises independently of the valve jumper. Once the head is off, you can try a couple of sharp taps to the valve body with a small hammer, just occasionally that can jar the jumper free, but often as not you have to drain down and remove the valve so that you can service it properly, at which point you might as well just replace it.
The rule is not to leave thermostatic radiator valves shut off all the Summer, since they are then forced closed and very often stick there, like this one has. Instead, open all your thermostatic radiator valves and just switch off the central heating at the programmer, or turn down the room thermostat if you don't have a programmer.
Sparks (the Plumber)