I'll update this thread when we know more but be sure to also keep an eye on the Blizzard CS EU Twitter feed.
If you believe you lost in-game items or gold from your account during this time, our customer service team is available to help you. Please submit a customer service ticket via the following link:
At this time we have no reason to believe that accounts currently using an authenticator are at risk. We've posted a corresponding message on the community website and will post further information here on the forums as it becomes available.