Did you remove your credit card or gift card info from your iTunes billing info? Maybe it's because of a problem with your billing info, if not your having traveled somewhere without the usual U.S. access to content. Other than that, I'm out of guesses.
If neither of those possibilities are at play, I would call Apple if your iPad is under AppleCare coverage or the first 90 days' free tech support with every iDevice, or take the iPad to an Apple store.