Recirculating loop stopped working

I did a gut regab in my apartment building 6 years ago and at that time put in a recirculating loop from the hot water heater in the basement to the fixtures on all 3 floors above. It worked great (without a pump), but suddenly it stopped working. Nothing changed, nobody touched anything. Have to wait forever now for hot water to come to the 3rd floor. How can this happen and what can I check? Any help is greatly appreciated. Thanks.

recirculation

Recirculation without a pump is a very tenuous situation and many things can stop it from functioning. One being a stuck check valve at the heater, but that is only one of many possibilities.