Q: music download interrupted, now iTunes app won't open or quit

Hello there, CoreyAmy.

The following Knowledge Base article shows how to quit an application that has become unresponsive:

OS X: How to quit an unresponsive application using Force Quit

http://support.apple.com/kb/HT3411

Additionally, this article offers some pointers on how to resume an interrupted download form iTunes:

iTunes: How to resume interrupted iTunes Store downloads

http://support.apple.com/kb/ht1725

If your download was interrupted using your Mac or PC

1. Open iTunes.
2. Choose Store > Check for Available Downloads.
3. Type your account name and password if prompted.

Thanks for reaching out to Apple Support Communities.

Cheers,

Pedro.