If you have the root password to your machine, you can recover the password for your Plesk admin login by viewing the Plesk password file. At the time of this article the password is still stored in plain-text accessible to the root user. Log into your server via ssh as root and run the following command:
cat /etc/psa/.psa.shadow
The output of that file is the Plesk admin panel, which can be used to login at the following URL: