Q: iv lost all music data

Hello Vic,

Welcome to the Apple Support Communities. If you are missing information from the library, the following will provide some assistance with rolling back the library file.

No content shows up in iTunes after updating

http://support.apple.com/kb/TS1967

Resolution

1. Quit iTunes.
2. Download and install the latest version of iTunes.
3. Use the Finder (Mac) or Windows Explorer (Windows) to go to the iTunes folder that contains the iTunes library files:

Mac OS X /Users/username/Music/iTunes/

Microsoft Windows Windows XP and Windows 2000 
\Documents and Settings\username\My Documents\My Music\iTunes\

Microsoft Windows Windows Vista and Windows 7 \Users\username\Music\iTunes\ 


4. Drag the iTunes Library file from the above location to the Desktop.
5. Open the Previous iTunes Libraries folder in the iTunes folder.
6. Locate the file named iTunes Library YYYY-MM-DD where YYYY-MM-DD is the date you upgraded iTunes (Year-Month-Day).
7. Drag this file to the iTunes folder (the enclosing folder).
8. Rename this file to iTunes Library.
9. Open iTunes.

You should now see your missing content in iTunes.

Regards,

-Norm G.