One of the causes of a cold radiator is trapped air which stops warm water from circulating properly; and the tell-tale sign is that your radiator will be cold at the top and hot at the bottom. However, it is an issue you can easily fix yourself by simply bleeding your radiator, although it is a good idea to do this once a year anyway.
Here is our how-to guide:

Turn the heating off and wait for the system to cool down as the water inside radiators can be very hot
Locate the bleed valve at the top of the radiator and put a towel or container under the valve to soak up any water that escapes
Place your radiator bleed key in the screw and turn it anti-clockwise about a quarter of a turn, or until you hear the hissing sound of air coming out.
Let all the air escape, mopping up any water that seeps out, and tighten the valve again (but not too tightly)
Turn your heating back on and your radiator will be warm all over, once again
 

If, on the other hand, your radiator is hot at the top but cold at the bottom, or if it has cold patches, there may be a build-up of sludge. In this case it is advisable to call out a heating engineer as they have specialist equipment to deal with this.