Air in the lines can prevent the water from circulating. In most systems, the pump is not strong enough to push the water up without some falling down the other side (i.e., the system is full of water). In this manner, they can use a smaller pump which uses less energy and is also quieter.
It's likely that you need to bleed the system.
Now, it's possible that the circulating pump has a problem, but try bleeding before looking at a hardware issue.