i have sync my iphone4 and it has deleted all my music and left me the purchased songs how do i get it back?

HEY

I HAVE A IPHONE 4 AND I WENT TO SYNC MY MUSIC (TRYING TO ADD A COUPLE OF TRACKS INSTEAD IT ONLY GAVE ME 38 TRACKS PURCHASED ONES) AND IT DELETED ALL MY MUSIC!! I HAD 400 SONGS AND TOOK ME AGES TO GET IT CAN ANYONE TELL ME HOW TO GET THEM BACK AS I HAVE TRIED EVERYTHING TO GET MY SONGS BACK.. THANKS!