iTunes won't update or open?

I attempted to update my iTunes on my macbook yesterday and apparently I didn't close the app before I started the update, so it gave me a warning message that in order to complete the installation I had to close iTunes...yet when I clicked on the app to close it I got another error message saying that I couldn't open iTunes during the updating. So, how exactly can I close the app if I can't open it? It looks like it's already closed itself when I started to update it but it keeps telling me it's open. The error message won't go away, either, and it's been on my screen ever since it popped up. Is there a way just to force-quit all my apps or something? Help please I need my music!