When a Nest thermostat's internal rechargeable lithium-ion battery dies completely due to power loss or a missing C-wire, the device will shut down its display screen, drop offline from Wi-Fi, and stop communicating with your HVAC system, leaving your heating and cooling disabled. On most models, you will experience offline alerts in your mobile app before it dies. To fix this, you must detach the display from the wall base and manually charge it using a micro-USB or USB-C cable plugged into a standard wall charger. Verification: Check that the display turns back on and battery voltage metrics rise above 3.7 volts after mounting it back on the wall base.
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