Need help: Songs and videos went missing yet Storage space taken up

[teof]

Hi guys,

I need some advice on this. I manage my songs and videos via manually adding. Last night I plugged in the iphone via usb and turn on Itunes. Suddenly, it showed my iphone has nothing inside. Every songs or videos was gone. However the space was still taken up....when I checked the storage meter...it showed over 12 gig taken up under Others. nothing under music or videos. Now I have barely 4 gig to work with. Does anyone know how I can rectify this prob? Thanks.