DIAGNOSIS The monitor is getting power but isn't receiving a valid video signal it can display. It's almost always the cable or the source, not the monitor itself.
FIX First, power cycle everything. Turn the monitor off at its button, then unplug it from the wall for 60 seconds. While it's unplugged, restart your PC or console. Plug the monitor back in, turn it on, then turn your source on. Make sure you're using a DisplayPort or Dual-link DVI cable (the ones that came with it are best for 144Hz), and check both ends are fully seated.
IF THAT DIDN'T WORK Try a different video output on your GPU. If you're on DisplayPort, switch to the DVI port using the right cable. This forces Windows or your console to re-detect the display. Also, try mashing the monitor's menu button—sometimes it defaults to a weird input.
HEADS UP This monitor doesn't have HDMI 2.0, so if you're trying to run 144Hz over HDMI, it won't work—you'll get a black screen or a capped 60Hz signal. For full refresh rate, you must use DisplayPort or Dual-link DVI.