Presuming this is a roof mounted, non-ducted, dial type thermostat on the A/C unit itself.
1. Confirm you have power up to the A/C unit itself.
2. Turn switch on and confirm you have power coming out of the switch to the A/C compressor and fan.
3. No power at the switch then the problem is from the main panel to the switch.
4. No power out of the switch-bad switch.
2006 23B Hybrid with 10k round bar WDH
2011 F150 4x4 SCREW Ecoboost, Max Tow, Integrated TBC, 3.73 LS axle, Firestone Ride Rite airbags.