Cold air blowing from return air vent

Cooling stopped working on my second floor zone, and cold air was blowing out of return air vent.

I turned off the power to HVAC , it didn’t work.

When I turned off the thermostat of my second floor zone and turned it on after an hour, to my surprise the issue was gone,
cooling started working in my second zone.

I am really curious to know how turning off the thermostat fixed the issue.

Hopefully these images helps to understand the issue