Diagnosis
The speaker is powered but has lost its logical connection to your Google account or Home app instance, often after a router reboot or app update.
How to Fix It
Force quit the Google Home app completely and reopen it. If that fails, reboot your phone/tablet. Then, power cycle the speaker: unplug it for 60 seconds, then plug it back in. Wait 2 minutes for it to fully reboot and reconnect, then check the app.
If That Didn't Work
The speaker is likely still linked to a different Google account or home structure. Have all household members check their Google Home apps. You may need the original account owner to remove the device from the home before you can re-add it.
Heads Up
If you're part of a Google Home with multiple members, only the person who set up the device (the 'owner') can remove it from the home. Other members will just see it as unavailable.