In order to obtain lost Plesk admin user access in linux for versions prior to and 12.5 you can use the following command in shell as root:
This is display the admin user password directly in shell. You can copy and paste it in Plesk login window. For newer Plesk versions than 12.5 you need to use the following in shell as root:
This is will generate a temporary login link which you can copy and paste in your browser. It will log you into Plesk as admin user and once you are there you can change its password.
For windows in case you have Administrator access you need to login via Remote Desktop Connection and then run as Administrator Windows Command Prompt. Type the following there:
This will bring a popup window with Plesk admin user’s password or you can also just use:
to set directly a new Plesk admin password.