Even if I go to the console, I can't login. (root and pswd)
From the blog.website.com/wp-admin, the login window is displayed but the login and pswd I've always used does not work. Nothing has changed. In fact I have not posted in over a month.
How can I reset the userid/pswd?
You can reset the root password for your droplet from the control panel. A new temporary password will be emailed to you. You'll then need to log in via ssh or the console and set a new permanent password to replace the temporary one.
Once you've regained shell access you should investigate. Your root password will not change on it's own and unfortunately WordPress sites are a prime target for attackers. You'll need to make sure your droplet is not compromised.
There are several methods available then to reset your WordPress user's password.