Do you want help or think that this a joke?
First move your itunes library to an external drive. After you do that, follow these steps.
Run either Tech Tool Pro 8, or Drive Genius to ensure that you have no bad blocks on your hard drive.
Having too many bad blocks can cause many problems such as yours. If you do have issues with the hard drive get a new one, if not follow along.
Defrag your hard drive.
Reinstall El Capitan the latest version.
After you do that proceed to delete itunes from your system.
Step 3 you will then reinstall the latest version of itunes.
Once complete restart and then open itunes holding the option key.
Then locate your itunes library on the external drive and it will work fine keeping your playlists and library intact.
After initial start up you will then have a fully functioning itunes with all your music intact.