Itunes shows no music on my device

It depends on where the music came from originally.

• If the songs are purchases from the iTunes Store, then you need to sign into your Store account (with the same user ID that you used to purchase the songs) in the new iTunes Library
• If the songs are from a different iTunes Library, then you cannot simply add them from the iPod into a new (or different) iTunes Library. Instead, you need to import the Library itself from the previous computer, or obtain them from the original source (CD or mp3 retailer etc.)

The only way to get music from the iPod into a different iTunes Library to its original one is to use third-party software. One usually needs to pay for the software.