There are no know or reported issues with iTunes in the U.S. at this time. You can always check here if you want to know.
What happens when you try to sign in? "It won't let me sign in" could mean a number of different things.
Have you tried resetting the iPad?
Reset the iPad by holding down on the sleep and home buttons at the same time for about 10-15 seconds until the Apple Logo appears - ignore the red slider if it appears on the screen - let go of the buttons. Let the iPad start up.Normally I would suggest signing out of your ID and then reset the iPad, but you are saying that you can't sign in.