I had the same problem since I downloaded the version 10.2.1. I had to use itunes urgently so I found a temporary solution until Apple updates to a newer version. I downloaded an older version of itunes (10.1.2.17) (
http://www.oldapps.com/itunes.php) later, I had to uninstall the version (10.2.1) and I removed (and saved) the file "iTunes Library.itl" in the folder Music \ iTunes, because it will not be compatible with older versions. Then I just installed the downloaded version. As I said this is a temporary solution, as you temporaly lost access to your iTunes library.
Then again, when installing the new itunes, replace the file "iTunes Library.itl" to the one you saved, to get your library back
sorry for my bad english, I'm catalan
greetings