If a Yahoo! Briefcase account has not been logged into for six months, it becomes "dormant."

When an account becomes dormant, it is deactivated, and the data contained in it is deleted. When an account becomes dormant and is deactivated, you no longer have access to it, and Yahoo! is unable to retrieve any of the data that was formerly stored in it.

Note: Premium Yahoo! accounts and accounts with Premium Storage are exempt from deactivation and data deletion.