Why is half my screen blue?

[qwert]

this just happened for some reason??

[qwert]

and now it just disappered???????

[zenzen]

Hi, and welcome to the ZorinOS forum.

This happens when you drag a window to one of the edges as a preview of where it will be placed. To be honest, I don't know how this could happen by accident... but it explains what's happening in your screenshot.