My Nest Learning Thermostat won't wake up and has a red blinking LED on the display
The Nest Thermostat has a rechargeable battery that gets its charge directly from your heating and cooling system. If the battery level gets too low, the Nest Thermostat will shut down until the battery is recharged.
Under normal conditions, the Nest Thermostat charges its battery from the low voltage wires on your heating and cooling system. But if there’s a wiring problem or an extended power outage, the battery may drain. When the battery drains below 3.6 volts, the Nest Thermostat will shut down and a red LED will flash on the display.
If you see this problem immediately after installation, it's possible that the Nest Thermostat's battery drained during shipping. This is extremely unusual, but does sometimes occur.
Once the Nest Thermostat is installed and the power to your heating and cooling system is on, the Nest Thermostat will start to charge. When the Nest Thermostat has sufficient battery, the display will turn on and the Nest Thermostat will begin setup. Unfortunately, while the Nest Thermostat is charging you cannot control your heating and cooling system.
The Nest Thermostat will usually take about half an hour to recharge. But if the battery is very low, it can take as long as two hours. If you're in a hurry, you can remove the display and plug it into a USB charger to charge the battery more quickly. This requires a USB cable with a USB Mini B connector (1st generation Nest Learning Thermostat) or a USB Micro B connector (2nd generation Nest Learning Thermostat) on one end.