Diagnosis
If fresh batteries didn't do it, the issue is either a stuck button on the remote or, more likely, the bar's infrared sensor on the front panel is blocked or the bar isn't fully powering on.
How to Fix It
First, check the obvious: make sure there's nothing physically blocking the small, dark IR window on the front of the soundbar. Then, unplug the soundbar from the wall outlet for a full 60 seconds. Plug it back in and let it boot up. This is a hard reset that can clear weird electronic hiccups that might affect the sensor. Point the remote directly at the front of the bar from about 5 feet away and try the volume or source button again.
If That Didn't Work
Your phone's camera can see the invisible IR light from a remote. Open your phone's camera app, point the remote's front end at the phone's lens, and press a button. If you see a faint white or purple light flashing on your phone screen, the remote works and the problem is with the bar's sensor. If you don't see a light, the remote is likely dead.
Heads Up
You can control basic functions (volume up/down, mute, source) directly on the top of the soundbar itself if you're stuck without a working remote.