How do I undo a sync???

With manually manage music and video selected, you can manually drag songs and video from your iTunes library to your iPhone.

Or deselect manually manage music and video and then sync your iPhone again - just as you did to get your iTunes content on your iPhone in the first place.

The iPhone backup stores data not included with the sync process - it does not store iTunes content which should remain on your computer.

With the computer you sync your iPhone with - any music available on your iPhone must remain in your iTunes library regardless if manually manage music and video is selected or not.

This is not what manually manage music and video means with an iPhone.

What happened to all your music that was in your iTunes library?

iTunes includes an option to transfer content purchased from the iTunes Store only from an iPod or iPhone, but if this content is no longer available on your iPhone, you need to restore your iTunes library from wherever you transferred the music that was in your iTunes library to.

Sorry, but your music is lost.

Selecting manually manage music and video with an iPhone after transferring music via the sync process erases the music on the iPhone, and the same in reverse.

iTunes includes a backup feature to backup your iTunes library and playlists, etc. to CD media, and there are other options for maintaining a backup of your iTunes library.

Accidental data deletion or on purpose is only one of the reasons for maintaining a backup of this and other important data. Your computer's hard drive could have a partial or complete failure at any time since none last forever.