DIAGNOSIS This is usually a backlight or power supply issue. The TV is on and working, but the LEDs that illuminate the screen have failed or aren't getting power.
FIX First, power cycle the TV. Unplug it from the wall, not just the remote. Wait a full 60 seconds. While it's unplugged, press and hold the physical power button on the TV (usually on the bottom right back panel) for 30 seconds. This drains any residual power. Plug it back in and try turning it on. If the screen flickers or you see a faint image, it confirms a backlight problem.
IF THAT DIDN'T WORK Check your picture settings. From a powered-on state (using the sound as your guide), try pressing the Home button on your remote, then navigate blindly: press Down once, then Right five times, then press Select. This might open Display settings. Then press Down twice and Select to toggle a setting like 'Energy Saving' or 'Adaptive Brightness' off. A factory reset (Settings > My Fire TV > Reset to Factory Defaults) can also clear a corrupted video setting causing this.
HEADS UP If the power cycle didn't help, this is often a hardware fault. The Omni QLED series has a known issue with backlight driver components failing. This requires a technician or a warranty claim; it's not a user-serviceable fix.