If you no longer have access to the Plesk web interface but you need to get the list of the ftp accounts than read the following tutorial.
By default all plesk data is locates in the psa database.
Connect to the mysql server
mysql -uadmin -p`cat /etc/psa/.psa.shadow` psa
The following mysql query will return the list of ftp users that were configured in Plesk subscriptions:
SELECT REPLACE(sys_users.home,’/home/httpd/vhosts/’,”) AS domain, sys_users.login,accounts.password FROM sys_users LEFT JOIN accounts on sys_users.account_id=accounts.id ORDER BY sys_users.home ASC;