DIAGNOSIS: A solid green light means the Arc is stuck in a factory reset or safe mode loop. It's not fully booting into its normal operating system.
FIX: You need to force a full power cycle. Unplug the Arc from the electrical outlet. While it's unplugged, press and hold the physical Play/Pause button on the soundbar itself (it's on the top, towards the right end). Keep holding it as you plug the power cable back in. Continue holding for another 5 seconds after the light comes on, then release. The light should cycle and eventually settle on white.
IF THAT DIDN'T WORK: If it goes back to solid green, the firmware might be corrupted. You'll need to contact Sonos support. They can push a firmware recovery file to the speaker if it's still visible on your network, which often requires a temporary wired Ethernet connection to stabilize it.
HEADS UP: Don't just unplug and replug without holding the button—that often doesn't clear the reset state. Also, if the light is green and *flashing*, that's a different issue (usually a network problem during setup). Solid green is the reset lock.