The hot water is working but the radiators will not heat up!
You did not give enough information (see my comment). But here is a guess for you anyway:

5 PSI is equivalent to 11.5feet of water. To me that means your radiators are filled to that height above the gauge and no more. (i.e. if the boiler is in the basement probably only the bottom of the radiator has water.) So no wonder they are cold - there is no water in them to carry heat.

But this is just a guess.