Iphone won't turn horizontally?

So i got my moms iphone 4 and the screen will not turn horizontally so i can watch videos on the whole screen. Is there a setting that i can turn off or something?

Just try to restart it and them it should work if that doesn't work the you shoul take it to the apple store

You can only take it to the Apple store if its in warranty. There's an orientation sensor that is going out on the phone. Its technically called an accelerometer. There's no fixing or repairing this. If you have a warranty Apple will give you a working phone. If no warranty, get on craigslist sell it As soon as possible before it gets worse.

Go to the home screen and double tap the home button and slide to the right and you will see a lock with a arrow around it make sure it's put on the right side

Try to scroll down touchscreen from up to down you would find the rotation button beside Wi-Fi speaker icon tap it its done