You may see this situation if you have both an Nokia Mail powered by Yahoo! account and a Yahoo! Mail account because you can only be logged into one of your accounts at a time since they are both running on the Yahoo! platform. 

Make sure that you have logged out of your Yahoo! Mail account, and then go to https://www.nokiamail.com to log in to your Nokia Mail account. Alternatively, if you have previously associated your Nokia Mail address with a Yahoo! account, you will be taken to your Yahoo! Mail account.