iTunes won't start up

This afternoon I was having a lot of trouble getting a playlist to synch correctly with my iPhone. Long story that I'll cut short here because it's a separate problem.

iTunes started hanging up after a while and I had to force quit twice. Now it won't even start up! The icon bounces in the dock for a while, then stops. No white line next to it, it doesn't even look like it's open, unless I Ctrl-click on it. Then I see "Application not responding."

What do I need to do?