what type of heating system do you have? is it a combi or a traditional type with a cylinder and tanks in the loft? if the radiators are not getting hot and you are having to bleed them constantly then the system may be sucking in air and needs looking at.
if you have a radiator that is completely cold and you have thermostatic radiator valves fitted then it could be that the trv is stuck. you can try taking off the head of the trv to expose the little pin, then spray some wd40 on the pin and try to loosen it by gently pushing on it with the end of a spanner or something like that, you can also try tapping the side of the valve under the pin to see if that helps.
you also need to make sure that the valve at the other end of the radiator is not closed off (very common) this will have a smooth conical cap on it which you need to remove and then you can use some small pliers or spanner to turn the valve, anti-clockwise for on, clockwise for off.