radiator not working?

i have just moved a radiator and now having problems with it. We hadnt used the old one for some time, so not sure if the existing pipework got blocked.
When all the pipework was done i tried it and the feed pipe got hot all the way to the rad. However the return is cold right back to the original valve and down under the floor again. The valve on the new rad is brand new so dont think thats the issue
Anybody any ideas how i can solve it ?
Thanks