DIAGNOSIS: Extreme Light Boost (ELMB) and variable refresh rate (like G-Sync Compatible) cannot run simultaneously. When ELMB is on, the backlight strobes, requiring a fixed refresh rate.
FIX: Press the monitor joystick. Go to 'GameVisual'. Ensure you are in a mode like 'Racing' or 'FPS'. Then go to 'ELMB' and turn it 'Off'. Go to 'Adaptive-Sync' and set it to 'On'. The brightness will return.
IF THAT DIDN'T WORK: If the screen stays dark, ELMB may have forced a low brightness setting. Go to 'Color' -> 'Brightness' and manually raise it back above 80.
HEADS UP: ELMB is only useful for very high frame rates (120Hz+). At lower frame rates, the strobe effect causes noticeable flicker and eye strain for many people.