A radiator knob is white and can be turned to the plus or minus position. If you radiator is cold, but the termostat is turned up, it can be that the radiator knop is stuck inside. This can happen for a number of reasons, but usually because it has not been used for a long time (e.g. summer).
When you disconnect the knob (behind it – in the black – you can see where the pin may be stuck. Depress the pin a few times and it should loosen it – WD40 liquid oil may also help.
Damage to this part of the radiator for whatever reason is the responsbility of the tenant. Once they are damaged it is advisable to replace them. If this is not possible, please ask a professional plumber.
If you decide to do it yourself however, take the faulty control to the plumbing shop so you know the correct one to replace.