I had the same problem, a couple of days ago, but solved ok, not entirely sure how though. It came about when I upgraded iTunes, which is definitely something I will NEVER do again.
It appears to have trouble whenever it does an upgrade. I am not sure if it has auto upgrade, as I am prompted to upgrade. I don't like auto updates, I always switch these off, particularly the Windows auto updates.
I also had the trouble buying from iTunes store. Even though it showed the correct card details, I deleted and did it again.
All working fine now. I should have known better, if it ain't broke, don't fix it.
Remember, anything to do with computers can and does go wrong. It is extremely complex coding after all.
I love Apple and iTunes. Just think what it was like without it. I just love the app store and have some fantastic apps.
Sure, I don't like everything about Apple, but I wouldn't be without my iPod and pad.