how to restore music back to ipod after syncing?

I have a 4th generation ipod. I connected my ipod to my PC when iTunes had a new version. As i updated, I didn't notice that the ipod was starting to sync and now all my music files are gone from the ipod. When I open itenes the files are still there, though I can't play them. Is there any way to restore those files back to the ipod again?

Please help!