If the Nest app reports a different power status than the Nest Protect itself, it's usually because of a delay in updating its status.
The Nest app may show that your wired Nest Protect's power is out for up to 24 hours after a brief power outage or fluctuations on the lines.
If Nest online services are available, your Nest Protect's power status should automatically update within 24 hours. If the status still hasn't updated after a day, or you'd like to update it right away, just press the Nest button on your wired Nest Protect once and it will connect to the Internet and update its status. You should see the status change in the Nest app within a few minutes of pressing the Nest button.
In rare cases, Nest online services may be temporarily unavailable, usually for scheduled maintenance. You can see the status of Nest services by visiting Nest's support site. Your Nest Protect won't be able to update until Nest online services are restored.
If the power status has not updated within a few minutes of pressing the Nest button, run a manual test on your Nest Protect. Press the Nest button once, it will say 'Ready, press to test'. Press once more to initiate a manual test. This will confirm the unit is working properly, and cause it to update its status in the Nest app.