has it got a pump?
that may have failed which is why the water isn't circulating

as for bleeding the rads, you need to first find out where the drain point is and drain the system first because the stuff coming out might explain whats going on, if it's all rusty to fuck water coming out then it needs some of that cleaner stuff bunged into tit

and yes, if you want to whip one of the rads out you need to drain the system first, and even then there is going to be a shit load of scummy shite coming out so plenty of absorbent cloth needs to go down either end when you unscrew them and make sure you don't let it drip onto the carpets as them stains ain#t never coming otu