could be the graphics card, graphic drivers, cable, or monitor.
The easiest thing to test is to see if switching monitor cables or ports fixes the issue.
next you could try software fixes such as running a virus scan and reinstalling your graphics driver.
If you still have the problem after the above you would need to test the graphics card and/or monitor to see which one is causing the problem.
Do you know what model the monitor is, what type of monitor cable and what computer & graphics card you are using?