Lost the sound on my Iphone 4S?

So everything was working fine, sound was on and full, not on silent and I was watching a video on my youtube app and all off a sudden my sound cuts out. Now no matter what app or video I play there's no sound to it, facebook, youtube, instagram, even my music on my phone doesn't play. When i go onto the music to try and play a song the volume bar is like in a grey colour and doesn't move when you click it.
Everything else plays, like my alarm or if I get a message or a call, it still makes sound but when I unlock and lock my phone the sound has gone on that too! Anyone know what to do?

Go to settings --> sound and check if everything is okay