ORA 01012 : not logged on

Hi,

When I tried to fetch data from dba_users table using command from ORASID it showing error as,

SQL> select username, account_status from dba_users;

select username, account_status from dba_users

*

ERROR at line 1:

ORA-01012: not logged on

Process ID: 0

Session ID: 0 Serial number: 0

SQL>

Kindly provide me the solution for this.