I have a Carrier Programmable Thermostat that was working perfectly until today! Out of the blue, there is no display on the thermostat! Don't know why or what would have caused it! What should I check first? I did check the circuit breaker and all was well.

Does it require batteries?

I have a programmable, but not same brand, that has to have 2 AA batteries installed for the display to show up.

It will work without them, but shows nothing on the display?

Actually this unit does not have batteries.