Riot Games have notified all EU West and EU Nordic & East League of Legends players warning them that their account details may have been compromised by hackers.
Some players' email addresses, encrypted account passwords, and dates of birth have been leaked. As a precaution Riot recommend you change your account password immediately by logging in here and clicking "my account" page in the top right.
A post from the President and CMO of Riot provides more detailed infomation. According to the post, "Even though we store passwords in encrypted form only, our security investigation determined that more than half of the passwords were simple enough to be at risk of easy cracking."
Riot have "fixed the security issue" and "hired experts" to make future breaches "less likely."
The post also offers a sincere apology for the breach: "We'll continue to invest in security measures, including password hashing and data encryption, state-of-the-art firewalls, SSL, security ninjas, and other security measures to make your info safer. We've been humbled by this experience and know that nothing guarantees the security of Internet-connected systems such as League of Legends. We can simply promise to try our very best to protect your data."