If it's of any help, on the LG G3, it's under Settings, General, Gestures....though I'm not certain that it's relevant to auto-rotate.
I have lost auto-rotate thrice on my G3....on the first two occasions, Soft Resetting fixed it. On the third, not so. I tried various things, but no go. Rukbat is quite right....when I looked in the settings, it was Off....though not my doing.
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