DIAGNOSIS The monitor is on the wrong input source, or the graphics card is outputting a resolution/refresh rate the monitor can't sync to.
FIX Manually select the input. Press the monitor's joystick. A menu pops up on screen. Navigate to the source icon (looks like a box with an arrow). Select the correct input (DisplayPort 1, HDMI 1, etc.).
IF THAT DIDN'T WORK Boot your PC into Safe Mode. This forces a low-resolution signal. If the monitor works in Safe Mode, your graphics driver settings are wrong. Reinstall your GPU drivers with a clean install.
HEADS UP This monitor requires a DisplayPort 1.4 cable for 240Hz at full resolution. A cheap or old DP cable will cause this error. Use the cable that came in the box.