Diagnosis
The wireless connection between the soundbar and the subwoofer has dropped or was never properly paired. The sub might be on but not receiving signal.
How to Fix It
You need to re-pair them. First, plug both the soundbar and subwoofer into power and turn them on. On the back of the subwoofer, near the power port, there's a small pairing button. Press and hold it for about 5 seconds until the LED on the sub starts blinking rapidly. Within 30 seconds, press and hold the 'Volume Down' and 'Input' buttons on the soundbar's top controls for 5 seconds. You should hear a chime and the sub's light will go solid.
If That Didn't Work
Move the subwoofer. It should be within 10 feet of the soundbar and not tucked completely behind a metal media console or a large appliance, which can block the wireless signal. Also, check that the subwoofer's own volume knob on the back isn't turned all the way down.
Heads Up
The initial pairing can be finicky. If it fails the first time, unplug both units from power for 2 minutes and start the entire process over from the beginning.