Hi JennyBrown57,
There may be a couple of ways to approach this. If the music on your iPod nano was purchased from iTunes you can get it back to your iTunes Library on your computer two different ways. You can either connect the iPod to the computer and transfer purchases or you can navigate to the purchased section in iTunes and download all of your content directly to your iTunes Library.
iTunes Store: Transferring purchases from your iOS device or iPod to a computer
http://support.apple.com/kb/ht1848
Download past purchases
http://support.apple.com/kb/ht2519
Thanks for reaching out,
-Joe