Diagnosis
The HomePod is stuck in a software boot loop or failed update state. The persistent spinning white light is the telltale sign.
How to Fix It
Force a restart. Unplug the HomePod, wait 10 full seconds, plug it back in. Immediately touch and hold your finger on the top of the HomePod. Keep holding until you hear three beeps and see a spinning red light, then release. This forces a reboot and often clears the glitch.
If That Didn't Work
You need to factory reset it. Unplug it, plug it back in, wait 10 seconds, then touch and hold the top. Keep holding through the white spinning light until you hear a second set of three beeps and the light turns red and pulsates. Release. Use the Home app on your iPhone to set it up as new.
Heads Up
The factory reset removes all data and settings. If it's part of a stereo pair, you'll need to re-create the pair after resetting both speakers.