You can always get the Plesk password if you have Administrative access to the server. If you have also lost administrative access to the server, refer to [How do I reset the password on my server]
For Windows,
1. From the Start menu, select Run.
2. Enter the following command:
“C:\Program Files\SWsoft\Plesk\admin\bin\plesksrvclient.exe” -get
For Linux,
1. Via SSH, run the following command:
cat /etc/psa/.psa.shadow