Hydronic Air Handler blows cold air

It's heating season but my HBXB-HW Series Hydronic Air Handler (made by first co.) stopped working. I have this unit in my condo but I get hot water supply from a boiler in my condo's basement managed by property. I can clearly feel the hot water is there because "water in" pipe is super hot - but to my surprise that the "water out" pipe is very cold. The hot water is not circulating!

I found an air bleed valve on the "water out" pipe side. I turned on this valve just to gave it a try and after collecting a bucket of water the running water became really hot - at this point the "water out" pipe turned hot. I restarted the handler but it still gave me cold air. About one hour later when i return to check the handler I can confirm that the "water in" pipe remained very hot while the "water out"pipe became cold again.

Refering to an old post I found a control valve on the heating hot water pipe near the handler's heating coil - but this valve is at on status and given the factor I can get hot water from "water out" pipe when i bleed air valve I believe there is no issue for handler to get hot water.

This unit was working last winter and I basically did not touch anything any valve since. The only thing I did was to turn it off (shut off electricity) in April as I went away from two months.

any advice will be greatly appreciated.