Try going over to the settings>sound>Device Vibration and see if the device vibration is enabled or not if its enabled and still your device doesn't vibrate then I am afraid but I think the vibrating motor set over your device board is damaged and needs replacement and for that you need to head towards the nearest customer support.
Does your phone app or your messaging app have an option to turn on vibrate? Try going into the phone app and hit the menu button. In settings, does it have an option for vibrate?
