Down link stopped, Uplinks still work

After more than a year of continuous operation, downlink messages stopped working last week. Uplinks still come through. Network server is 3.12.1. I have Class A and Class C downlinks, both do not work via API and manually enqueued messages via the Chirpstack console. I have stopped and restarted the 3 processes, and did a Ubuntu server reboot. Will appreciate some help with tracking down the issue.

Trace through the sequence of things that need to happen for a downlink to happen and see at what stage it’s failing. Notably, are the gateways getting sent transmit requests? What do logs show?

I am also facing same issue regarding the DOWNLINK data.
The Sensor is connecting with the Server.
Also able to send the Data from Sensor to the Server(UPLINK).
Then trying to receive the data from the Server to Sensor,
But found that, the Downlink is not working.,
because the frame count is not incrementing.

So did you face any such errors while working?

My Chirpstack version is 3.16.2.

Please reply,
it is very urgent.
Thank you in advance.