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OK are you sure batteries are new alkalines and installed properly(+/-)? Display should come up immediately even if no power to unit
If batteries are new and properly installed,Set System switch to Off. Remove batteries and insert backward for five to ten seconds to reset thermostat. Replace batteries correctly; display should come on.
Let me know if this helps
Did you try the backwards thing?
Did you try the backwards thing?If you did and still no display, you will have to replace thermostat as it has undoubtly taken a power spike.