Manual synchronization from a Mac to an iPod such as the iPod classic is one-way: from computer to iPod.
The iPod can only be synchronized with one Mac at a time. When you do a synchronization, the music on the iPod is replaced with whatever selection of songs, from the Mac's iTunes/Music library, that you told the Mac to put on the iPod (assuming there is enough room).
If you had a bunch of songs on the iPod, and you did a new synchronization, and the iPod classic now only has two songs in its library, you will not be getting back the previous songs from the iPod, itself. They have been deleted or overwritten. You will need to go back to the iTunes/Music library on your Mac. If you did not keep that library safe, you might need to re-download music purchased from the iTunes Store and re-import music that you had imported from your own CDs.