Welcome to the Apple Support Communities. if iTunes has become unresponsive, it is good to wait it out to see if it becomes responsive again. If it does not, then the next steps would be to preform a Force Quit on iTunes. The following article will assist with that process.
To force quit an application, choose Force Quit from the Apple menu (or press Command-Option-Esc), then choose the unresponsive application in the Force Quit window, and click Force Quit.
Optionally, you may be able to Option-right click the application's icon in the Dock and choose Force Quit from the contextual menu.
Additionally, you can force quit the front-most application in Mac OS X v10.5 or later by pressing Command-Option-Shift-Esc for three seconds.