Why has my messaging tone and vibration stopped working?

[AC Question]

On my Xperia E1 the notification sound and vibration have stopped alerting me to incoming messages. I've looked up the settings on the messaging app and the relevant notification tone is checked and works fine. How can I get the sound and vibration to alert me to incoming messages agin, please.

 

[B. Diddy]

Welcome to Android Central! Do other app notifications work normally? Is it only when you have the Messaging app opened to the conversation where you don't get the notification, or even when you're on a homescreen or when the phone is asleep?