how can i fix a radiator?

you can first check that the valves either side of your radiator are open,
if they are you may need to bleed the radiator,
on older radiators there should be a square shaped plug in one end at the top, newer ones also have a slot for a screw driver,
open this slowly and only partially, if it has air in it you should hear the air vent out, do this until water starts to flow out have a small tub to catch the water, you can buy bleeding kits at most diy stores, the radiator should then start to heat up nicely

where is the radiator in the system, upstairs or downstairs? furthest or nearest?

Rusko
November 2009