From the official twitch.tv blog:
TL;DR: DON’T PANIC – We were not hacked. Our web CDN made a requested change without obeying our caching ruleset, which resulted in some caching that had a (very, very) slim probability of revealing a limited amount of your account information. To be cautious, we’re changing stream keys and requiring a password reset on your next login. Finally, no payment information was exposed as we do not store any of this information.
We’ll continue to update regarding when the site will be restored, but we wanted to make sure that you change any passwords on other sites that are the same or similar to the password you use on Twitch.