Apple's system status page is currently reporting a fairly big iTunes Store outage, with 20% of their customer base shown to be affected. Typically this means the iTunes Store itself, along with the App Store and iBookstore, are inaccessible for many, many people.
Every service goes down every once and a while, and iTunes, while fairly reliable, is no exception. If you're having trouble, know that you're not alone. Let us know if you're having problems, what type, and if it starts working again, let us know when.