How do I fix a TV where the sound works but the screen is black?
Model: LG 65" Class 4K Ultra HD LED Smart TV - 65UH615A - 350124
Have a peek through the small holes in the metal shielding to check in there is any backlight. If there is backlight, then the T-CON board is likely to be faulty.
If there is not backlight, shine a strong flashlight very close to the screen and look very closely, you may be able to distinguish some picture. In that case, the backlight is at fault.
With such symptoms, I would rule power supply and motherboard issues.