Weird problem with iTunes? When I sync, doesn't show All my song files?

I have synced my iPhone with iTunes with no problems so many times in the past and on my computer it has all the same files that are being transferred onto my iPhone from iTunes exactly (this includes all music, videos, etc.). But I just opened iTunes and now it only shows my iCloud music purchases. All my other song files that I downloaded and put on iTunes are gone from my computer. However, all of them are still on my phone but if I sync, it will delete all the songs not showing on the iTunes list. What do I do? Is there a way to recover my original library with all the songs (I have around 600 songs in my library, but this one only shows my iCloud song purchases, there's only like 10 of them)

You can follow this iphone transfer guide to copy songs, music and video files from iphone to computer http://www.001-software.com/resource/how-to-transfer-music-video-from-ipodiphoneipad-to-computer-and-itunes.html