We have a hot water pipe going from the boiler to an air-handler in the attic, then return to the boiler. The recirculating pump is located on the return side. In the last few days the attic got very cold (below freezing point) and I guess the water in the pipe or the hot water coil of the air handler was frozen, since the hot water inlet of the air-handler was cold and we got no heating. Since then the temperature in the attic has increased to above freezing point for a couple of days and I think the frozen water must have been thawed. But the water is still not recirculating through the pipe and we still got no hot water to the air-handler in the attic. The recirculating pump is working but is only pumping air - since the pipe above the recirculating pump is empty! (I checked by opening the relieving valve above the recirculating pump, no water is flowing out)
How do I push the water through the empty pipe? Since the re-circulating pump is on the return side it doesn't help.
Any help will be greatly appreciated!

Do you have hot water flowing to the inlet side of the coil? If so check around the coil and the air-handler for leaks a tube in the coil may have broken when it froze.

Thanks for the suggestion! There is no hot water flowing to the inlet side of the coil. It appears that the hot water from the boiler can't get up to the air-handler in the attic, and the pipe returning from the air-handler to the boiler is empty. It got empty probably because the recirculating pump was pumping the water into the boiler when the pipe in the attic was frozen. Now I wonder how I can push the hot water up to the attic. The cold water inlet of the boiler has a pressure reducing valve so the pressure is probably not enough to push the water up.