ORA-01012: not logged on

$ sql

SQL*Plus: Release 11.2.0.4.0 Production on Fri Mar 6 16:44:14 2015

Copyright (c) 1982, 2013, Oracle.  All rights reserved.

Connected.
SQL> startup mount
ORA-01012: not logged on

Solution : Shutdown Abort the database and start it again to solve ora-01012

Connected.
SQL> shutdown abort;
ORACLE instance shut down.
SQL> exit 
Disconnected

$ sql

SQL*Plus: Release 11.2.0.4.0 Production on Fri Mar 6 16:52:19 2015

Copyright (c) 1982, 2013, Oracle.  All rights reserved.

Connected to an idle instance.

SQL> startup mount
ORACLE instance started.