2 Answers

When Yahoo Email Accounts Closed For Any Reason

There Is No Way To Restore It

Examples

( Yahoo Email Accounts Are Deactivated And Removed After Four Months Of Not Signing In The Account )

( Yahoo Deleting The Account )

( Deleting The Account Yourself )  <<<<<<<<<<

When an account is deactivated, you won't be able to access it, regardless of whether or not email has been received in the account during that time.
And sorry, but you can't retrieve any of the information that was formerly stored in it.
You will lose access to your Yahoo! ID, Yahoo! Mail, and profile names. yahoo will also delete your account data and settings across the Yahoo! network. This includes any contents in:
Yahoo! Mail
Yahoo! Address Book
Flickr
Yahoo! HotJobs
GeoCities
Yahoo! Briefcase
My Yahoo!
Yahoo! Groups
Finance Portfolio

NOTE
( Or any other area of Yahoo! that can only be accessed with your Yahoo! ID and password )

Info From The Account Deletion Page

This Applys To Accounts Deleted For Any Reason

In most cases your account will be deactivated and then deleted from yahoo user registration database in approximately 90 days.
This delay is necessary to discourage users from engaging in fraudulent activity. Additionally, due to the limited number of names available,
we (MAY)allow other users to sign up for and use your current Yahoo! ID and profile names after your account has been deleted.
it is (NOT) automatic that you can get or anyone can get that user id again 

Source: Yahoo e-mail help center.