Why won't my iPhone 4 ring or light when it's locked?

When it's unlocked, it rings, vibrates and i get the notification but when it's locked won't vibrate, ring, or light up.
help?

In my opinion, there are only 2 possibility's really. (1) it's on "do not disturb" go onto the settings and do not disturb should be right there:) just under half way down the screen so you shouldn't need to scroll down or whatever at all, make sure it's flicked OFF not ON:)
or (2) it's broken haha