I recently moved into a new place which has two floors. It is heated with hot water and the heater gas-powered heater is in the lower floor. There are two radiators in the upper floor that stay ice-cold, while one, in another room also upstairs is warm. I am unsure if the problem is just a failure to properly turn them on or something else. The two cold radiators are different from the others in that they don't have knob to turn them on. I have attached a picture of what it looks like. There are two pipes connected to them but they are also always cold, and vanish into the wall. I called the previous owner who built the whole place but he is very old and doesn't remember anything about it. How do I start diagnosing what's wrong?
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.