Diagnosis
The wireless link between the soundbar and the subwoofer has been lost.
How to Fix It
Power on the soundbar. On the subwoofer, find the ID SET button on the back. Press and hold it for 5 seconds until the red LED blinks rapidly. The soundbar will search and reconnect. You should hear a test tone.
If That Didn't Work
Perform a full system reset. Unplug the soundbar and subwoofer from power. Press and hold the soundbar's Volume (-) and Input buttons on the unit itself for 10 seconds. Then plug everything back in.
Heads Up
The subwoofer must be within 33 feet of the soundbar with minimal large metal obstructions in between. Concrete walls with rebar can also block the signal.