Welcome to the Apple Support Communities. Before you go through the process of importing your music from your iPod, check to see if the music is already on your computer. Check in the Following path C:\Users\username\My Music\iTunes\
If there is media there, then import that into your iTunes library.
If you need to transfer your music back from your library, you can transfer purchases that have been made through iTunes. This article will assist with that process.
You can alternatively download previous purchases directly from the iTunes store.