How can fix this on my iPhone 4S?

When I turn my iPhone 4S to the side to watch a video, it won't turn sideways. How can I fix this?

To turn it on or off, do the following:

In iOS 7 or higher, swipe up from the bottom of the screen to reveal Control Center. In iOS 6 or lower, double tap the home button on the front of the iPhone or iPod touch. This will reveal the fast app switcher row of apps at the bottom of the screen
In iOS 7, tap the screen rotation lock icon. If it is white, it's on; black and it's off.
In iOS 6 or earlier, swipe left to right to reveal the Music app, music player controls, and on the far left, an app with a curved arrow in it (if the lock is turned on, you'll see the lock inside the arrow)
To turn screen rotation lock on, tap the curved arrow icon so that the lock appears
To turn screen rotation lock off, tap the icon so that the lock disappears
When you're done in either version of the iOS, single tap the home button and you'll be returned to normal operation of your device.

Try going into multitasking (double tap the home button) swipe as far left as possible, you should see a play button and some others loon for the one with a lock and a circle. Try tapping that you may have portrait orientation lock on which won't allow your phone to rotate, hope this helps!

You might have the screen locked. To fix this, double click your circle button until a task menu appears. Then, slide it to the right one time and you should see a circle arrow with a lock in the middle. Press that and the lock should go away. Then you can turn your screen.