There are two possible reasons that an account can be deactivated:

• Yahoo! Mail accounts are deactivated and removed after six months of not being used, plus an additional two months for each year you held the account.. An account is considered dormant if it hasn't been accessed for that interval, even if email has arrived during that time. Note: Signing in via an email client, such as Microsoft Outlook, Mac Mail, or a mobile phone mail application is not considered a valid login. You must sign in to your account on a Yahoo! web site using an Internet browser (Internet Explorer, Safari, Firefox, Chrome).
   
• A violation of the Yahoo!'s Terms of Service. Please read the Terms of Service for complete information.

Once an account has been deactivated, it's impossible for the Yahoo! Mail team to retrieve any stored information.

Note: Yahoo! Premium Mail accounts are protected from dormancy, and are not subject to deactivation due to inactivity. Please visit our Yahoo! Mail Plus page to learn more and sign up. Note that Yahoo! Mail Plus is not available in all regions.