Ubisoft's online systems were hacked, and its account data were compromised. The illegally accessed information includes user names, email addresses, and encrypted password. The publisher is asking all users (if you've ever signed up for Uplay, that's you) to reset their passwords via this page.
Ubisoft clarified that it doesn't store any personal payment information on a support page, so don't worry about canceling credit cards. But it would be a very good idea to change the password unilaterally if you used it for any other services.
Ubisoft says it is investigating the unauthorized access with relevant authorities and security experts. Its website remains closed for maintenance at the time of this writing.