Kindle won't play audio books?

I got a Kindle as a present. I loaded several audiobooks onto it and they worked fine. Recently, the screen was broken and I was sent a replacement, with all my books reloaded. I just tried to play one of the books, and it told me I couldn't play it because my Kindle wasnt authorized with Audible.com. If that hasnt been a problem before, why is it now? PLease help!!