According to a Kotaku report, Valve's Steam service suffered a security breach which allowed attackers to easily gain access to a number of user accounts over the last week. According to Valve, the bug has since been fixed.
The company provided this statement to the website:
To protect users, we are resetting passwords on accounts with suspicious password changes during that period or may have otherwise been affected. Relevant users will receive an email with a new password. Once that email is received, it is recommended that users login to their account via the Steam client and set a new password.
Please note that while an account password was potentially modified during this period the password itself was not revealed. Also, if Steam Guard was enabled, the account was protected from unauthorized logins even if the password was modified.
We apologize for any inconvenience.