Diagnosis
The speaker is in setup mode, likely triggered by a recent factory reset or a network configuration glitch. It's waiting for you to connect it via the Google Home app.
How to Fix It
Open the Google Home app, tap the + icon, and select 'Set up device'. Choose 'New device' and follow the on-screen instructions to reconnect your speaker to your Wi-Fi. This is a standard re-pairing process.
If That Didn't Work
If the app can't find the device, perform a forced factory reset. Press and hold the center of the touch panel on top of the speaker for about 15 seconds until you hear a sound confirmation. The light will pulse orange, then go solid. Start the setup process in the app again from scratch.
Heads Up
After a factory reset, all your personal settings and speaker pairings (like a stereo pair) are erased. You'll have to reconfigure everything.