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Hi I'm Brian and I'm here to help!
Just to review, the thermostat was blank and dark and when you put batteries in it, it booted up correct?
Ok, that tells me that there is no 24 volt power coming into the thermostat from the air handler. Even though you checked the breakers, it could still be a couple of things....
It shouldn't be wired to do this but if you have a drain line or drain pan overflow switch and the installer wired it so it shut everything off, it could do this so I would check that first. The switch is usually in the drain line right at the unit and it has two wires coming out of it. You can pull that out of the fitting and check the float for sticking and the line for a clog. If there isn't one in the line and the unit is laying horizontally with a metal pan underneath it, there could be a pan switch there. You will usually see that the pan is full of water. BUT... these switches are USUALLY wired to just stop the ac from running, not the fan in the air handler or power to the thermostat so this is a reach........
The next most likely thing is a low voltage fuse has blown in the air handler. This is usually a small 3 or 5 amp automotive style fuse on the air handler or furnace control board that's behind the blower panel mounted to the blower housing. If that's blown it will give the same symptoms.....
That is all assuming you indeed are getting correct incoming voltage to the air handler. So the short check list is:
1) Breakers to the air handler. Shut off then back on.
2) Float switch, drainage issue.
3) Low voltage transformer fuse in the air handler.
After that, it's time to get the meter out and find the issue probing for voltages.
Thanks, ***** *****! Let me know if I can help further!