yes!  you will lose all your music, so be careful. you may want to set your ipod on manual mode if you don't want it to automatically sync with a new or an empty itunes library.

If you still have your music in your ipod, what I can really recommend is to put it back into itunes. Just download this tool, select your music and clic backup to itunes! That's all! It will automatically import all your songs, playlists, videos, tv shows, etc. and restore your library!