To unlock Yahoo account, you have to navigate the Yahoo mail sign-in page. After that, you must enter your username and password to enter. If it requested to follow the verification steps, then you must do it. After this verification, your account will be reactivated and you can use it.
https://www.number-directory.com/blog/unlock-yahoo-account/

Step 1: Go to Yahoo Help Forum Page.

Step 2: Here click on “Account Locked” message

Step 3: Now click on Sign-in Helper

Step 4: Enter the Yahoo Mail Address or Phone Number (You can even enter recovery email ID or Phone No).

Step 5: An ‘Account Key’ will be sent to your recovery mail/phone or you may be asked for the security questions.

Step 6: Follow the steps and you will regain access to your locked Yahoo! Account instantly.

When you log in to your account, cookie in the form of packets of data are stored on Yahoo servers. These cookies are the one which helps recognise your Yahoo account for further references. But when someone attempts a sign in using the wrong password the cookies are deleted. And when the limit number of attempts are crossed the cookies expire. This stimulates the Yahoo filters to think account activity as suspicious and thus locks it temporarily or for so called 12 or 24 hours.
To unlock or unblock your account you can try to Log in using another Yahoo server. These results in the formation of new and fresh cookies. The new server does not have any knowledge of expired cookies. And you are able to login and access your Yahoo! mail account again.

Hope this helps.