Music/playlists missing

Not sure how this one happened - yesterday everything in iTunes is okay, but I open it today and all of my content (except content purchased on iTunes) is missing, and so are all of my playlists. Everything is intact on my iPod Classic, though. Is there any way to easily copy/move all this from my iPod back to iTunes? It's probably easier than it seems but I'm not tech-savvy enough to figure it out. Any help is much appreciated.