DIAGNOSIS The Nest is not receiving power from your HVAC system's C-wire or the internal battery is depleted. This is a common installation fault.
FIX First, pull the thermostat display straight off the base. Check the wire labels on the baseplate. You must have a wire connected to the C terminal. If not, you need to install a C-wire or use the Nest Power Connector accessory. If a C-wire is connected, ensure it is secured under the terminal screw.
IF THAT DIDN'T WORK The internal rechargeable battery may be fully drained. Connect the Nest's Micro-USB port to a standard phone charger for at least 2 hours. Reattach to the base after charging.
HEADS UP Without a C-wire, the Nest charges its battery by briefly turning on your heating system. In mild weather with infrequent HVAC cycles, this can lead to a recurring low-battery blank screen.