DIAGNOSIS The video stream is failing, but the audio and connection are fine. This is often a problem with the app or your phone struggling to decode the video, not the camera itself.
FIX Start with the simplest fixes on your viewing device. 1) Force close the Google Home app completely and reopen it. 2) Restart your phone or tablet. 3) If it's still black, check for an update to the Google Home app in your device's app store and install it. 4) Finally, try viewing the camera feed from a different device, like a computer at home.nest.com. If the video works there, you know the problem is isolated to your primary phone.
IF THAT DIDN'T WORK If the video is black on all devices, the camera's image sensor might have failed. The last resort is a factory reset of the camera (hold the button for 10 seconds). If after a reset and re-setup the video is still black, you'll need to contact support for a potential hardware replacement.
HEADS UP Before you do a factory reset, make sure you know your Wi-Fi password and that you're okay with losing any custom activity zones or face labels you've set up. The reset wipes the camera back to factory defaults.