If your default instance is in trouble, and you can not use the normal login process to reach the server, you can use a special login to log into the server without any instance. Here is how to use SQL*Plus to log in as as a system BDA:

>cd (OracleXE home directory)

>.instartdb

>.insqlplus

Enter user-name: SYSTEM/globalguideline AS SYSDBA

Connected to an idle instance

SQL> show instance

instance "local"

The trick is to put user name, password and login options in a single string as the user name. "AS SYSDBA" tells the server to not start any instance, and connect the session the idle instance.

Log in as SYSDBA is very useful for performing DBA tasks.