Diagnosis
The TV has a picture processing issue, most commonly a failed backlight system. The sound plays because the main board and streaming components are working, but the LEDs that illuminate the LCD panel are not turning on. Less often, the issue can be a fault on the T-Con (Timing Control) board or a loose internal cable.
How to Fix It
First, perform a power cycle to reset the hardware. Unplug the TV from the wall outlet. Find and press the physical power button on the TV itself (often on the bottom or rear) and hold it for 20 seconds while it's unplugged. Release the button, plug the TV back in, and turn it on with the remote. If the screen remains dark, shine a bright flashlight at an angle on the screen in a dark room. If you can faintly see a menu or image, the backlights are definitively dead.
If That Didn't Work
If no faint image is visible even with a flashlight, the problem may be with the T-Con board or its connections. This requires opening the TV's back panel. Carefully check that the ribbon cables connecting the T-Con board to the mainboard and the LCD panel are securely seated. A damaged T-Con board would need replacement. If the backlights are confirmed dead, the LED strips behind the LCD panel must be replaced, which is an advanced repair.
Heads Up
Replacing the backlights involves removing the extremely fragile LCD panel and diffuser sheets. Any pressure, flexing, or a single crack will permanently ruin the screen. Diagnosing internal boards is safe, but the actual backlight repair is best left to a professional technician.