Hi Loa, there are a couple of troubleshooting things you can do when an app starts to misbehave for no apparent reason.
Try deleting you preferences for that app and then relaunch it. In this case:
com.apple.iTunes.plist
com.apple.iTunesHelper.plist
(if you are partial to your personal settings, it's best to drag these prefs to your desktop and add something to the name eg com.apple.iTunes_old.plist. If your problem turns out not to be corrupted preferences you can reverse the process to restore your prefs.)
Check to see if iTunes works on another account on your Mac. This will at least tell you if you have a problem with your app, or something wrong with your assets. (I have a spare account on my Mac just for these occasions.)
Check on Apple's support knowledge base to see if this has happened for others.