This is usually because the TV stopped communicating with the Arc for whatever reason.  You can usually resolve by one of the following steps.

• Check your TVs audio settings and make sure CEC is turned on (may be under a different name, but point is that TV knows to send audio through ARC/eARC port.
• Unplug the HDMI cable between TV and Arc for a few seconds, replug.
• Unplug the TV for several seconds (a cold reboot), then replug.

Try this:

Disconnect everything from the TV and unplug the TV and Arc from power for a couple of minutes. Plug everything back in but keep everything disconnected. Make sure CEC is enabled in the TV settings. Then run TV Setup in the Sonos app and follow the instructions. When the app instructs you, connect the Arc to the TV using the Sonos-supplied HDMI cable. After the Arc has been successfully connected, connect any other devices to the TV.

It might be a few things. Here’s some initial things to perhaps consider …

• Has CEC control been switched off on the TV?
• Has TV autoplay been switched off in the Arc Room settings in the Sonos App? 
• Have you tried running through TV setup in the Sonos App room settings?
• Did you try pressing the TV icon at the bottom of the Music Services Tab to ensure the TV audio was selected for the Arc room?
• Have you checked the HDMI cable connections and reseated the cable?
• Have you tried power cycling both the TV & Arc?