My iPhone screen won't turn the display horizontally?

My phone won't turn horizontally. When I text or watch a YouTube video or basically anything else the display remains vertical even when I turn it. Why?! I've tried turning my phone on and off. I can't watch YouTube videos with a large screen or turn my phone to use the large keypad to type. It's an iPhone 5. Do any of you know how I could fix this? I'm so frustrated! It's been like this for about a week. I tried updating it and that didn't fix it.

Your phone is probably on Lock Rotation. If your phone is on iOS 7, swipe up to get the control bar, and click the little lock button. Also, check the top right corner of your screen next to the battery icon and see if there's a lock there. If there's, that means your phone is locked on Portrait mode.
My iPhone screen won t turn the display horizontally