How to enable the ring/mute button on my iPhone 4?

In class, one of my friends thought it would be funny to turn off the mute on my phone. He didn't break the button, but when its on ring or mute, it always rings. I had to go the rest of the day with it ringing in all my classes -.- I told him to fix it but he said he forgot how to.

He didn't open my phone because he doesn't know my password (at least I hope he doesn't) so how could he have possibly disabled my mute button? The only access that he would have while my phone is locked is the flashlight, volume, clock, calculator, camera, and the other things on the white thing that slides up. There's no option for the ring/mute button, is there?!

Anyways, I've been looking everywhere in my settings and I can't seem to fix my problem, can anyone here please tell me how to enable my silence button? Thank you.

P.s: again, the button is NOT broken.

Flip the side switch.

Go to Settings/Sounds/"Change With Buttons". Toggle that and the switch if that doesn't work, he broke your phone. Only setting in iOS 7 to change that…