Installed El Capitan and iTunes stopped working

Please quit iTunes if it's running, then drag the icon up and out of the Dock. Open the Applications folder and drag iTunes back into the Dock. Any change?

This procedure will delete certain temporary and cache files. The files are automatically generated and don't contain any of your data. Occasionally they can become corrupt and cause problems such as yours.

Please back up all data.

Triple-click anywhere in the line below on this page to select it:

/var/folders

Right-click or control-click the highlighted line and select

Services ▹ Open

from the contextual menu.* A folder with the odd name "folders" should open.

Inside "folders" are several subfolders, each with a two-character name. Drag all the subfolders except the one named "zz" to the Trash. Don't delete the subfolder named "zz". You'll be prompted for your administrator login credentials.

Restart the computer and empty the Trash.

*If you don't see the contextual menu item, copy the selected text to the Clipboard by pressing the key combination command-C. In the Finder, select

Go ▹ Go to Folder...

from the menu bar and paste into the box that opens by pressing command-V. You may not see what you pasted because a line break is included. Press return.

If you don't already have a current backup, please back up all data, then reinstall the OS.* You don't need to erase the startup volume, and you won't need the backup unless something goes wrong. If the system was upgraded from an older version of OS X, you may need the Apple ID and password you used.

There are ways to back up a computer that isn't fully functional. Ask if you need guidance.

If you installed the Java runtime distributed by Apple and still need it, you'll have to reinstall it. The same goes for Xcode. All other data will be preserved.

*The linked support article refers to OS X 10.11 ("El Capitan"), but the procedure is the same for OS X 10.7 ("Lion") and later.