How do I get all my music from my iPod and copy it onto iTunes again?

I had to reboot my computer and for some reason my iTunes library deleted every single music/video file that was originally on there and only has some of the music on there. Thankfully I still have everything left on my iPod with nothing deleted but I need help copying everything again onto the library without using any trial program online where it restricts you from copying everything back on there. Do you think it's even possible to even do that? Please let me know...