Diagnose cold radiator
That looks like the body of a Thermostatic valve (TRV). It looks like the actuator pin (in the center of the front face) is stuck in. This can usually be unstuck by wiggling with a pair of pliers. If it's really stubborn, some WD-40 or a LIGHT tap with a small hammer may help. You can then get a new TRV head and add that to control the room temp. There should be another valve on the other end of the radiator and it's usually worth screwing that closed and then re-opening it.