thats as far as my knowledge goes, so take this as a bump
most of the time its because of monitor cable, happened to me, screen tearing, flickering etc. then i changed my cable from hdmi to dp and everything worked fine. what is your cable type?
Did you check settings? Maybe you somehow resettet to smth lower than 240Hz?
Check in windows and in screen HUD.
I can bet my life savings that it's a GPU issue
do you have integrated graphics or a spare GPU? try using that instead of your main one and see what happens
do you have 2 monitors? I get a lot of screen tear when my 2nd monitor is turned on, I usually turn it off when playing cs
How is this the fault of your monitor though?