Hey xyridion,
It sounds like you've moved the "Render" window in ClickGUI off your screen, but in modern versions of Wurst you can't actually do that anymore. Instead, the "Render" window will show up as a thin line at the one of the edges of your screen, where you'll be able to grab it and put it back in a normal position.
Here is a screenshot to show what this can look like on all 4 sides:
In this example, I dragged the render window off-screen on the left side while it was open, creating a long line almost all the way across the left edge of the screen, which I think is quite hard to notice. So if something like this is what happened to your render window, it might have been right there all along and you just never noticed it.
Let me know if you find it. If not, you can always delete your .minecraft/wurst/windows.json file to reset all ClickGUI windows instead.
Edit: Also remember that to move a ClickGUI window, you have to grab it at the top part where the title bar is. Easy to forget when it's just a thin line on the side like that.
