Diagnosis
The wireless connection between the soundbar and the subwoofer has been lost. They need to be re-paired.
How to Fix It
Here's the re-pairing sequence. 1) Unplug the power cords from both the soundbar and the subwoofer. 2) Wait 60 seconds. 3) Plug the subwoofer back in. You'll see its status light glow. 4) Now, plug the soundbar back in. It will automatically search for and pair with the sub. You should hear a low-frequency test tone come from the sub when it connects, and its light will go solid.
If That Didn't Work
The subwoofer might be too far away or obstructed. Make sure it's within 10 feet of the soundbar and that there's a relatively clear line of sight—thick walls or large metal objects can block the wireless signal. Also, check that the sub's driver (the big circle on the side) isn't physically damaged or punctured.
Heads Up
Other wireless devices, like powerful routers or cordless phones operating on the 5.8GHz band, can occasionally interfere. Try moving the router or temporarily turning it off to see if the sub reconnects.