DIAGNOSIS You're likely using the wrong cable or have it plugged into the wrong port. This monitor's 144Hz refresh rate is only available through its DisplayPort connection or one of its two HDMI ports (usually labeled HDMI 1).
FIX First, ensure you're using a capable cable. 1) Use the DisplayPort cable that came in the box, or a certified high-speed HDMI cable (like HDMI 2.0). 2) Plug it into the correct port on the monitor—try the DisplayPort first, or the HDMI 1 port. 3) On your PC, right-click the desktop, go to 'Display settings' > 'Advanced display settings' > 'Display adapter properties for Display 1' > 'Monitor' tab. Here, you should see a 'Screen refresh rate' dropdown. If 144Hz isn't listed, select 120Hz as a test. If higher rates appear, you've confirmed the cable/port is the issue.
IF THAT DIDN'T WORK You may need to enable the high refresh rate in your graphics card control panel (NVIDIA Control Panel or AMD Radeon Software). Look for 'Change resolution' and set the refresh rate there. Also, some older graphics cards or laptops can't output 144Hz over HDMI; check your GPU's specs.
HEADS UP If you're using a VGA cable, you will be locked at 60Hz. VGA is an old analog standard and cannot support high refresh rates at 1080p on this monitor.