How do I retrieve iTunes that disappeared from iPod after syncing it?

The system is designed so that you can only sync an iPod against one master library, on one Mac or PC, at a time.

If you sync with a new computer, or a new library, that wipes all of the existing music on the iPod in favor of music from the new master library. That's how it was set up to work from the very start.

As I said, the system is designed to let you sync against only one master library at a time. If you loaded this iPod classic with music "for a long time", it is very likely that you were using a different computer, or at the very least, different installation of macOS, when you loaded it.

Connecting it to your MacBook Pro a few days ago would have been seen as a sync against a new library, so when you synced it, that would have wiped any existing music off the iPod classic.

The idea is that you are supposed to safeguard the music in the master iTunes or Music library on your computer. If you do that, then the tunes may "disappear completely" from your Pod when you first sync to a new computer, but you can re-load them from the music library that you safeguarded and transferred to the new computer.

kennethfromhuddersfield wrote:

Are they retrievable or they lost for ever?

If you don't have them on your computer now, but they were purchased from the iTunes Store, you can

Redownload music from the iTunes Store on your Mac - Apple Support

If you don't have them on your computer now, but you imported them from your own CDs, you could

Import songs from CDs into Music on Mac - Apple Support

You'll need an external optical drive.