Hello, what to do when UV Image Editor is completely gray? In edit Mode, everything is selected. 
I was rotating a UV map of one of the wall faces so it would look better in Unity, but then it became completely gray. Restart didn't help.

HAALP!
You have probably hiden the UV Editor grid accidentally. To restore it hold the Mouse Scroll and drag the mouse left/right/up/down. You may also scroll the mouse wheel to make it appear. Another method is pressing the Home button.