Honeywell Programmable Thermostat Heater "t8011r battery" Questions

I have Honeywell T8011R programmable Thermostat. Turn AC on last night for first time...working fine...came back late morning to find display and fan off. Check circuit breaker...on. I took it off and

...it worked for me. I woke up one morning and my thermostat was blank,the thermostat is a honeywell T8011R and does not require batteries.My system will not turn on,I need some suggestions on what to ...

battery backup? I have a programmable Honeywell Thermostat T8000C or T8001C or T8011R. The thermostat has been working fine EXCEPT when I've lost power. I think there must be a battery backup to keep ...