In Ubuntu 17.04 there was 'Rotation' menu in Display tab. But now in new version of Ubuntu, there's still a Display tab but no more rotation menu. Now I can't turn that off again since I upgraded to 18.04 LTS.
You may use the keyboard combination Super+O to toggle the rotation lock ON/OFF as described in @Sylvain's answer.
Note that Super is usually the key with Windows logo.
What do you mean "turn off screen rotation" ? Do you mean go back to normal orientation ?
Anyway, in the new gnome-control-center of gnome 3.28, in Devices > Screen Display tab, you can click on Orientation and a menu appears to let you choose between
- Landscape - Portrait right - Portrait left - Landscape (flipped)
I guess this is what you want ?
for me none of these work. As soon as I put the computer to sleep and wake it up again, it puts itself in the wrong orientation (due to the fact that the computer is sitting on a tilted support that raises it up). The only thing that I manage to do quickly is
xrandr -o normal
to fall back to normal orientation. This is painful