I've lost my whole library after syncing new music to ipod

the rest of my library has gone!

I hope you mean the rest of your library is gone from the iPod, but it is still there on your computer. If all of your songs are in your iTunes library, and you want to put "everything" on the iPod (there is enough storage on iPod to fit everything), this is how you do it.

Select iPod in iTunes, so that you see its settings screen in iTunes window. Along left side of window, in the sidebar, under Settings, click Music. To the right, you see the iPod's Music settings screen.

NOTE: Any songs on iPod that are NOT in your iTunes library (on the computer) will be lost.

At the top of window, checkmark the box for Sync Music. Below that, select the option to sync Entire music library. At the bottom, click Apply. Going forward, iTunes keeps iPod updated with changes to your iTunes music library, when you connect it ti iTunes (or click Sync if already connected).

(If your iTunes music library does NOT fit in the storage capacity of your iPod, please post back.)

I'll describe how to add additional songs to what is already on the iPod, using the manual method. What I described before may not have worked, if your complete iTunes library is too large to fit on your iPod. First, select the iPod in iTunes to show its Summary settings screen. Checkmark the box for Manually manage music [and videos], if it's not already. At the bottom, if you made a change, click Apply.

Then in iTunes, on the horizontal bar that goes across the window, at the left end, click the Music button to show your iTunes music library. Near the middle of the bar, click Playlists. A sidebar appears along left side of window, with your iPod listed under Devices. At the far right end of horizontal bar, click the control to change the view of music library to Songs. This shows the music library as a plain list of songs.

Select a song that you want on iPod. Drag it to the sidebar and drop it on the iPod. This loads that song on iPod. You can do this with more than one song selected at a time. OR, instead of dragging and dropping, right-click on selection and Add to Playlist -> [name of iPod]. If you want to add complete albums, you can change the view to show your library by Albums (instead of by Songs). Then drag albums to iPod.

To see what is currently on iPod, click the small triangle to the left of iPod's name in the sidebar, to drop down its content list. Click Music there to show the list of songs on iPod (to the right).

NOTE: When the iPod is set to use the manual method, you need to Eject the iPod in iTunes before physically disconnecting it.

Is all of your music in your computer's iTunes library? If so, you can sync it (all or a portion) back onto the iPod.

Songs loaded on iPod need to be stored locally on the computer. The cloud means it is only stored in your iCloud music library. To download song, just click on the cloud.