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Hi I'm Brian and I'm here to help!
The most common reason for this (as long as the system worked before starting) is the low voltage fuse or transformer in the indoor unit or packaged ac unit has blown. Can you tell me the make and model number of the indoor unit or packaged ac unit (whichever you have) please?
Oh, ok. I thought when you said neither worked that you may have put the old one back on and it didn't work either. In that case it could be a wiring issue or it could be the thermostat face did not mate properly with the thermostat wiring base. Those are the most common if the 24 volts is still intact. Of course there's always the possibility the new thermostat is bad out of the box (don't ask me for the standard acronym for that condition ;) ).
A common issue today, especially with WiFi thermostat's needing both 24 volt hot (R) and 24 volt common (C), is there is no C connection at the thermostat wired as is to you indoor unit. Many thermostats don't need 24 volt common connection but most WiFi thermostats do. If you're experiencing the same issue with two thermostats, Bad out of the box seems very unlikely.
Feel free to pick this back up when you get home and we can get to the root of it together. I do take a dinner break or I may be assisting another customer so please forgive a slow response. I usually work until midnight EDT so I'll be around!
If you can put your hands on a volt meter, that will be really helpful finding the problem. I'll look for your next post later. Thanks, -Brian