Went away for few days. Returned and now only upstairs AC working. Downstairs thermostat
(Honeywell) keeps blinking "Cool On", but doesn't come on

Do you have 2 ac units or do you have a zoned system?

Zoned system with one unit. I went to the circuit breakers and now the thermostat (only on the floor not working) has the word 'recovery' on it, but is still not working. Doesn't seem to need new battery as it is working in that it shows correct date, time, etc.

Did you turn off power to your main control that runs your damper system? This will reset the board and sometime correct the problem. Damper systems can be very hard find what the problem is. If you can not find the problem with the wiring you can open the damper manually

System may take up to 10 minutes to come back on after you cycle the power