Welcome to the forum This isn't a riddle is it? Is there not a setting for it any more or is it set to auto rotate now but won't?
There is a switch/icon in the notification area at the top of the screen which allows you to lock or unlock the rotation. I think it defaults to lock. If that doesn't work Go into Settings. Go into Accesability (in the 'System' section) Check the auto-rotate screen option.