The data accessed included email addresses and encrypted account passwords, and they have determined that more than half of the passwords were simple enough to be at risk of easy cracking.
If you are a League of Legends player and happen to use the same password for your League of Legends account and your Battle.net account, we strongly encourage you to reset your Battle.net password at this time. To complete a password reset, go to Account Management > Settings > Change Password. You can also visit this page if you've forgotten or lost your Battle.net password: https://eu.battle.net/account/support/password-reset.html
Additionally, if you used your Battle.net email address to sign up with League of Legends, you may also wish to update your Battle.net email address via Account Management as a general security precaution. To update your Battle.net email address, go to Account Management > Settings > Change Email Address.
If you are unable to update your password or email address and believe your account might be compromised, please contact our customer support staff by using the Account Recovery Form:
For additional security tips and suggestions, be sure to check out the Account Security Awareness guide at http://eu.battle.net/en/security/