If you need to reset the password, you must first SSH into the server, once logged in use this command (replace "ThisIsMyPassword"):

plesk bin admin --set-admin-password -passwd 'ThisIsMyPassword'

For more information on how to use this utility, please view the official Plesk documentation here: https://docs.plesk.com/en-US/onyx/cli-linux/using-command-line-utilities/admin-administration-panel-preferences.38401/