Thanks for using Apple Support Communities. I would recommend trying a thorough removal and reinstall of iTunes as described here:
With regards to the music in your library, the article states this:
- iTunes Store purchases or songs imported from CDs are saved in your My Music folder by default and aren't deleted by removing iTunes. While it's highly unlikely that you'll lose any contents of your iTunes Library when following these steps, it's always a good idea to ensure that your iTunes library is backed up. If you're unsure how to backup, follow these steps.
In general, if you ever need to download past iTunes purchases (note this does not apply to items not purchased through iTunes), this article has information on how to do that: