Cold Radiator

I've got a radiator that will get a little warm, but not hot like the others. What might be the problem? I see there's some sort of valve on the side, but I don't have the right tool to do anything with it. I can probably borrow some needle-nose pliers if turning the valve is the thing to fix it. Suggestions? Thanks.
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If its at the top and you have hot water, not steam its a bleeder valve, open it till water comes out while your system is running
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