Need more info to help you out. Analog or digital monitor? do you have a extra monitor you can plug into your PC? If so, swap the monitors to check to see if it’s the video card and not the monitor that is the problem.
check the physical wiring and connections. Also check to see if you have moved your speakers or any other interference causing device closer to the monitor.
someone may have adjusted your clock & phase setting on your analogue monitor
that’s the main difference between a digital & analogue monitor.
site that explains it
http://www.techmind.org/lcd/phasexplan.html
use this to set your analogue LCD’s phase.
http://www.techmind.org/lcd/phasing.html
Interesting site, thanks.