That's some other symptom so I won't say it's that. If the screen backlight has failed, you could do the usual flashlight test such as https://www.youtube.com/watch?v=xUzCLgoSPGM

That's a very complete video on LCD TV basics. You as the TV repair tech would want the service manual and know about the lethal internal voltages so with that out of the way the service manual is key for great diagnosis. That is you use the manual to check test point voltages and follow the service diagnosis trees.

Good hunting,
Bob