What i have to do with my iphone?

I have an iphone 5 and this 2 days the message doesn't have sound. But when someone calls me it has. Only message sound is off, i don't know why.

Added (1). Switch on the upper left side is turn on.

Added (2). Yes it doesn't show red. And the volume buttons is up but it still doesn't.

There's a switch on the upper left side. Is it turned off? That mutes the sounds. You want no red showing.

The other thing to check is the volume buttons on the side. With the phone idle, press the up button to adjust the volume of the ringer.