DIAGNOSIS This is almost always a simple input or source selection issue. The bar is powered on but it's either not receiving an audio signal or it's looking for sound from a source you're not using.
FIX First, check what input source the soundbar is set to. Press the 'Source' button on the remote. A small light will show on the bar's top—white for TV (eARC/ARC), blue for Bluetooth, green for Wi-Fi, etc. Make sure it matches where you're sending sound from. If you're using the TV, the most reliable method is HDMI eARC/ARC. Go into your TV's sound settings and set the audio output to 'HDMI ARC' or 'eARC' and turn off the TV's internal speakers. Power cycle both devices by unplugging them for 60 seconds, then plug the TV back in, wait a minute, then plug the soundbar in.
IF THAT DIDN'T WORK The HDMI cable might be the culprit. Try a different, high-speed HDMI cable that you know works. Also, double-check that the soundbar's volume isn't muted (the light ring will pulse white when you press volume down if it's at zero) or turned down very low.
HEADS UP If you're trying to use Bluetooth, remember you have to put the soundbar in Bluetooth pairing mode (press and hold the Bluetooth button on the remote) and connect from your phone. It won't automatically grab a Bluetooth signal like a portable speaker might.