Diagnosis
The wireless pairing between the soundbar and subwoofer has been lost. They need to be re-paired.
How to Fix It
Power on both units. On the back of the subwoofer, locate the small 'Wireless' button. Press and hold it for 5 seconds until the blue light starts blinking rapidly. Immediately, press and hold the 'Wireless' button on the back of the soundbar for 5 seconds. Wait. The subwoofer light should turn solid blue when paired.
If That Didn't Work
Perform a full reset. Unplug both the soundbar and subwoofer from power for 5 full minutes. Plug the soundbar back in first, wait 30 seconds, then plug the subwoofer in. They should auto-pair. If not, repeat pairing steps.
Heads Up
Keep the subwoofer within 10 feet of the soundbar during pairing, with a clear line of sight. Thick walls or large metal objects can block the 5.8GHz wireless signal.