I returned home from a couple days away to only one of two a/c units working. The display is completely blank on my honeywell t8000c unit downstairs - the second floor is working just fine. Of course, I checked the breakers - neither were tripped. Where do I go from here? I don't know jack about a/c units. Our house is only 2.5 years old... so I am pretty sure it is not due to wear and tear just yet... any suggestions? Thanks!

It could be the
Thermostat itself
Thermostat battery
The 24 volt control transformer located in the inside air handler
The low volt wiring from thermostat to indoor unit
Blown fuse on the circuit board in the inside unit.

That should give you enough places to check

It should be one of the above.