Can you regain your older music back on your iPod?

My playlist and most of my music is either greyed out or not showing up on my iPod anymore. But it still shows all the songs I have on my iPod on iTunes but none of them work. This all happened since I was on somebody else's computer. I have an iPod/iOS 6.1.6. I have tried to turn off the iPod it didn't work, rebooting the computer didn't help either. I tried to even do "Restore Backup", "Sync Music" to get them back and it just ending up deleting all of my iPod's custom made playlists, but the music is all still showing up on iTunes. What can I do? Please help.