To reset the Plesk admin password on Windows servers, from the Plesk bin folder %plesk_bin% run the command plesksrvclient.exe
"%plesk_bin%\plesksrvclient.exe"
It will produce a pop-up box in which you can enter a new password. You will then be able to login to Plesk as admin using the new password.
You can also see the existing password using the command below
"%plesk_bin%\plesksrvclient" -get