If a radiator remains cold (when the central heating boiler is working properly) or just the top of the radiator remains cold, this suggests there is an airlock in the radiator. An airlock in the radiator is always at the top and will block the water flow. This can be solved by bleeding the radiator.
You will need a radiator key and an old towel or cloth. Radiator keys are available at any hardware store.
If none of the above gets the heating working properly, please leave a repair request.