The backlight has a few parts on sets but you left out the model number so I will only broach the LED models.

Google "How to test HDTV LED STRIPS" and you know how to test for failed strips.
Along the way you'll have picked up that there is usually only one board that powers the LED strips so if that is suspect you replace that.

Please folk, always include full model numbers with the trail 4 or 5 letters and model year.