Touch a hot soldering iron to it so it goes back to a liquid, then remove the iron so it cools again. A 'cool' or 'dry' joint is where the solder hasn't actually melted around the wire/pad, only pretty close. It's usually caused by something moving while the joint is cooling, not having the solder hot enough, or the contact surfaces not being cleaned properly (although flux usually helps with this). Reflowing it usually fixes the problem.
Vostro A90 1.6GHz/2GB/32GB SuperTalent 10.6.6
Mid 2009 MacBook Pro 2.8GHz/4GB/1TB+500GB, 10.6+Win7
Mini 1012 1.66GHz/1GB/160GB Win7 Pro