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!
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Sign up to join this communityYou 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.