STARTUP command fails : Why is that?
To restart Oracle, connect via SQL-PLUS as SYSDBA as you did previously, and type in the commands

shutdown normal;
startup mount;
alter database open;
exit;


Oracle SQL Developer can help you graphically administer the Oracle Database.

Most Oracle metadata are stored in special tables. For example,


To show all tables: select * from cat;
To show all users/schemas: select * from dab_users;
To change schemas: alter session set current_schema=NEW_SCHEMA_NAME;


Are there any other specific Oracle commands you were looking for?