I am unable to open the Material Editor no matter how many times I doubleclick on the material. Even after restarting my computer and reinstalling Unreal Engine, the issue persists. The editor window just doesn’t appear.
Has anyone experienced this before or does anyone have any suggestions on how to resolve this?
Any help would be greatly appreciated. 
Thanks 
I’ve had this happen once. The window is off screen for some reason. Here’s how I fixed it, but not sure it’ll work for you but worth a shot.
Hover over the Unreal Editor icon in the taskbar and wait. A popup like the one in your screenshot will show up. Move the mouse over the Material popup and right click. Then a small menu should come up and click Maximize. This should make the material window go fullscreen. Now, you’re not out of the woods yet. If you close, it could revert to where it was when you open it again. So drag the titlebar of the Material window until it’s on your screen normally (not maximized). Then close it so that it remembers where you have it. Once you open it again, it should be back to normal.
If this doesn’t work, you can try changing your screen resolution for a bit to see if it resets your window location.
And if that doesn’t work, you can press “Windows Key”+Tab. This will open the multiple desktop screens. You can then drag Unreal Editor to another desktop and it might reset the window position. Then you can move it back to the original desktop.