That happend to me too.
Always when Rylai cast her ulti my monitor turned off and sometimes it happend randomly.
And i couldn't turn in back ON so i had to restart PC.
But after i runed warcraft on -opengl it never happend again.
1: Right click on Euroloader
2: Properties
3: In the target field add -opengl
It should look like this: "C:\Program Files\Warcraft III Reign of Chaos & The Frozen Throne\euroloader.exe" -opengl
It will 100% fix your problem and even increase FPS and make the game more stamlbe if you have a old PC.