Diagnosis
No lights at all means the bar isn't getting power. Since you've tried outlets, the issue is likely the bar's own internal power supply or, more simply, the detachable power cord connection.
How to Fix It
First, check the cord where it plugs into the back of the soundbar. That DC connector can work loose over time. Unplug it firmly, inspect both ends for any bent pins or debris, and plug it back in with a solid push until it seats completely. Listen for a click if there's a locking mechanism. Also, trace the full length of the power cord for any obvious cuts or severe kinks.
If That Didn't Work
The power supply (the “brick” on the cord) might have failed. If you have a multimeter, you can check if it's outputting the correct DC voltage (it should be labeled on the brick, likely 15V). If you don't, the only real test is to try a replacement JBL power adapter with the same voltage and connector size. Don't just use any old adapter—the voltage and polarity must match exactly.
Heads Up
Before you assume the worst, try plugging the bar directly into a wall outlet you know works (like one with a lamp), bypassing any power strips or surge protectors. A faulty strip can be the culprit.