ITunes freezes when I hit "authorize," and why am I suddenly having to authorize?

I've been on ITunes for ages. Today, when I tried to play something with a "cloud" icon, I was told my computer wasn't authorized to play it. I got the latest version of ITunes, but when I try to authorize, ITunes repeatedly freezes. I have only one computer here. Why am I suddenly having to authorize? How can I prevent the freezing?