Itunes wont transfer music from my iphone to library

Heres the story:

I got my iPhone 3G back in april. I then got my new macbook back last august. So up until a few days ago or so, it would work fine, all of my music on my library would show and so on. Well sometime not to long ago, something happened (maybe my phone was plugged into another computer), and now when I plug my iphone into my itunes, it shows my iphone there, but empty library and everything gone off there. If I want to sync my music to itunes, its saying it will erase everything on my iphone. The only thing its letting me tranfer to my Itunes is my music I purchased from itunes on my phone and my apps. but all of my regular music wont transfer to my itunes library. Any help?

I cant even drag and drop my music from my iphone to itunes library since its not even showing the actual songs on my phone. The reason im trying to do this is because I just got a windows phone 7 (samsung focus) and want to put my music from my iphone on my new phone.

Thanks