How are you trying to view the Yahoo? In the Mail App? You can add it back in... And you can always log in on browser.
Open Mail > on the left margin click "Accounts" > in the right popup panel click "Add Account" and enter the credentials > the Yahoo will now appear in the left pane and may be selected to view from the list of accounts.
If you forgot your password due to automatic login being your habit... That is stored in Control Panel. In the search box on taskbar type in "credential" and click the "Credential Manager Control panel" result > "Web Credentials" tab > scroll to the Yahoo entry > select the down arrow to reveal details > "show" > you will have to input your Admin Password (probably the password you log onto the machine with) in order to reveal what the password you use for Yahoo is...
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