music library cleared when updated

This article should help

http://support.apple.com/kb/TS1967?viewlocale=en_US&locale=en_US

Another approach...

Empty/corrupt library after upgrade/crash

Hopefully it's not been too long since you last upgraded iTunes, in fact if you get an empty/incomplete library immediately after upgrading then with the following steps you shouldn't lose a thing or need to do any further housekeeping. In the Previous iTunes Libraries folder should be a number of dated iTunes Library files. Take the most recent of these and copy it into the iTunes folder. Rename iTunes Library.itl as iTunes Library (Corrupt).itl and then rename the restored file as iTunes Library.itl. Start iTunes. Should all be good, bar any recent additions to or deletions from your library.

See iTunes Folder Watch for a tool to catch up with any changes since the backup file was created.

When you get it all working make a backup!

tt2

Each time iTunes updates it archives the old library file and creates a new one. If the update process fails it can create an empty library but your media should still be there, replacing the current libray file with an earlier copy as described above will trigger a new library update that will hopefully be sucessful this time, listing all your media, playlists, ratings etc.

tt2

You will be able to redownload any purchased itunes store music by

Slecting the Purchased link from under the Quick Links section on the right of the iTunes Store page

Select your music and redownload.

If the music is still on your device

This article from Turing may help

https://discussions.apple.com/docs/DOC-3991

Or you may have to re import from CDs

The old libray files will normally be in <User's Music>\iTunes\Previous iTunes Libraries. Or have you updated computers as well as iTunes versions?

tt2

If by updated you mean a complete rebuild of Windows wiping the hard drive then yes, otherwise your old profile folder with your iTunes library may be preserved somewhere.

tt2