When a radiator is cold at the top or not heating up at all it could be a sign that there is air in the system. To remove the air you will need a bleed key.
Locate the air vent on the radiator, engage the bleed key and gently turn anticlockwise until you can hear the air coming out. Using a paper towel or an old cloth to catch the drips, when the water starts coming out turn the key clockwise taking care not to over tighten.
You may need to top up the pressure on your boiler, please refer to manufacturers instructions on how to do this.