What is wrong with this iPhone? - 1

My sister has an iPhone 4 iOS 7 and she's had a problem with it where she can't tilt her phone when she's watching videos on youtube or on anything like safari. We've tried to restart it and we've also checked if it's locked. It's not, but it still won't tilt. Does anyone know what's wrong with it? It kinda happened unexpectedly, so we don't know what could have caused it.

Maybe you have the potrait orientation mode on. In the picture I posted you see the lock? If it's white like mine just press on it & it should turn black. Now you will be a able to tilt.
What is wrong with this iPhone - 1

Yes check at the top if you have the oriental thing on you can turn it off