It's possible that due to multiple attempts of logging in or password changing, your account might be locked temporary. This happens to prevent other unauthorized logins. For example, even if you changed the password, someone else could still try to log-in, therefore triggering the flags and locking the account.
Make sure you set a strong password of upper case letter, special characters, numbers and symbols to make it hard for anyone to guess it.
If the issue persists in a day, contact the Yahoo support team and submit your request to it. Use the following page, please: https://help.yahoo.com/kb/helpcentral