This is a common problem which people have with their heating system, there are a few simple thing you could try before calling a heating engineer.
Radiator warm at the bottom but cold at the top
This usually happens when you have had your central heating off for a long period of time and caused by air in the heating system.
Use a radiator key to "bleed" the air out.
Using a radiator key open the air vent on the top of your radiator slightly. Allow all there air to get out (you should hear a hissing sound”. Once the “hissing” stops and water starts to come out of the vent close it again. The tops of your radiators should now get hot.
Please Note; If you are using a combi boiler system you may have to re-pressure your heating system. You can see how to do this here
All radiators warm except one cold all over.
If you have one radiator not getting hot there is a problem with the water circulation. Check that both of the radiator valves are open.
You can also try closing a few of the other radiators to push the hot water to the cold radiator. If this works your system need balancing and you should contact a local heating engineer.
This could also be a sign of a blockage. You will need to call a professional as you may require you heating system power flushed to remove any debris and blckages.
All radiators cold?
Is the boiler actually working? Check that the your gas is working or if you are using a pre-payment meter make sure you have enough credit.
If you have a combi boiler check that the pressure on the pressure dial is in the range as specified in the instruction manual and top up with water if necessary. You can see how to refill your boiler pressure here.
All radiators only a little warm but not hot enough.
Could be of any many causes. Your need to get a professional boiler engineer in.