It makes life difficult when so many people hijack a thread!
However, if a rad is cold then obviously hot water is not getting thru it!
If the rad is hot at the bottom and cold at the top then you might have air trapped in it, so bleed it.
If the whole thing is cold, either its sludged up, or something else is stopping water flowing into it. Are both valves (incoming and outgoing) open? If they appear to be, check the pipes leading to them coming from the floor.
If the incoming one is hot, but the rad cold, then the rad valve at one end is faulty. It might be a TRV, in which case the pin might be stuck down. You'll have to take off the TRV bit (the big head usually just unclips from the valve itself or is attached by a biggish knurled ring that unscrews) and you'll see a metal pin. Is it stuck inside the valve or is it sticking up? Try pressing on it with a flat-bladed screwdriver. It should be springy up and down. If stuck down, you'll need to pull it out with pliers. It only moves about 5mm in either direction, so dont go mad with it. If you can shift it upwards, does the rad start to warm up? If so, squirt some WD40 so it soaks into the valve around the pin. Give the pin a few pumps up and down - it should spring out when pressed in. If the rad is warming now, then put on the TRV head again, pat yourself on the back and warm your smug toes on it whilst thinking of the cash you've saved by not calling a plumber!
If still not working, either the valve as a whole is kaput, which is a bigger job, or the valve at the other end is faulty, or you dont have enough hot water pressure in the system to force it thru the rad. On a combi boiler system check your presure gauge and if it reads way less that 1, that might also be the problem, and if so, come back a say so. On a gravity system (with a header tank in the loft) check the header tank is not empty.
I would stress that I am not a plumber, just have had lots of problems with my rads too, so I feel your pain. A bit of detective work is needed to isolate the problem.
Hope this helps a bit.
Or, call a plumber