This morning my son (3 year old) was watching some video on my PC. He kept pressing keyword - suddenly the screen started looking like this...
I couldn't find how he managed to change (some) setting. Re-starting Windows didn't help. Kannan Balasubramanian helped me. Thanks Kannan! (Sustaining saves the day again!).
Steps to fix "tilted Windows":
For some reason, while Kannan was able to make it "Normal" and immediately view the change, I had to re-boot! Anyway, now it is "Normal"