I would recommend starting with the Concepts and Examples guide.
https://www.juniper.net/techpubs/software/screenos/screenos6.3.0/630_ce_all.pdf
As for the password issue, ScreenOS only allows one root user. You can have multiple read/write users, but they do not have the same ability as the root user. You cannot "remove" the root user, only change the username. It sounds as though someone changed the root user to your user, then changed it back.