It may happen that after updating iTunes some of your music files, videos or podcasts no longer appear in your library. This procedure explains how to clear your iTunes library and create a new one, by deleting two files from the iTunes folder.
First close iTunes and then navigate to the iTunes folder:
On Mac OS X: Click on Go > Home. Navigate to Music > iTunes.
On Windows: \Users\username\Music\iTunes\.
Delete the iTunes Library.itl and iTunes Music Library.xml files: