DIAGNOSIS: The HDMI connection between your TV and the Arc has probably lost its handshake, a notorious HDMI-CEC/ARC/eARC issue. The data signal is there, but the audio path is dead.
FIX: Perform the full HDMI reset. Unplug both the Arc and your TV from wall power. Unplug the HDMI cable from both ends. Wait a full 60 seconds. Plug everything back in, starting with the TV power, then the Arc. Let the TV boot completely before turning the Arc on. This forces a fresh negotiation between devices.
IF THAT DIDN'T WORK: The HDMI cable itself might be the culprit, especially if it's a cheap one not rated for high-bandwidth eARC. Swap in a certified 'Ultra High Speed' HDMI cable (the ones that often come with new game consoles are a good test).
HEADS UP: Some TVs have a specific setting to enable the eARC/ARC port. Dig into your TV's sound or external speaker settings and make sure 'eARC', 'ARC', or 'Audio System' is explicitly turned on, not just 'Auto'.