It keeps on turning, is your Auto-rotate screen option is turning on itself?
As mentioned by @Tallman, Please check Auro rotate option is enabled and check. Follow the below steps:
Swipe down the screen to access your Quick Settings and tap on or Landscape to change your screen rotation settings. By selecting Auto Rotate, you will easily be able to switch between Portrait and Landscape mode. If you choose Portrait this will lock the screen from rotating to landscape. To lock the screen horizontally you will first need to switch to Auto-rotate, turn the screen into landscape view then select Landscape.
