ORA-03113: end-of-file on communication channel

SQL> startup
ORACLE instance started.

Total System Global Area 8589934592 bytes
Fixed Size                  3731384 bytes
Variable Size            1140850760 bytes
Database Buffers         7432306688 bytes
Redo Buffers               13045760 bytes
Database mounted.
ORA-03113: end-of-file on communication channel
Process ID: 1264
Session ID: 814 Serial number: 23355


SQL>

[ora@sujeet ~]$ sqlplus / as sysdba

SQL*Plus: Release 12.1.0.2.0 Production on Thu Aug 22 21:07:49 2019

Copyright (c) 1982, 2014, Oracle.  All rights reserved.

Connected to an idle instance.

SQL> startup nomount
ORACLE instance started.

Total System Global Area 8589934592 bytes
Fixed Size                  3731384 bytes
Variable Size            1140850760 bytes
Database Buffers         7432306688 bytes
Redo Buffers               13045760 bytes
SQL>
SQL> alter database mount;

Database altered.

SQL>
SQL>
SQL> alter database clear unarchived logfile group 1;

Database altered.

SQL> alter database clear unarchived logfile group 2;

Database altered.

SQL> alter database clear unarchived logfile group 3;

Database altered.

SQL> shutdown immediate
ORA-01109: database not open


Database dismounted.
ORACLE instance shut down.
SQL>
SQL>
SQL> startup
ORACLE instance started.

Total System Global Area 8589934592 bytes
Fixed Size                  3731384 bytes
Variable Size            1140850760 bytes
Database Buffers         7432306688 bytes
Redo Buffers               13045760 bytes
Database mounted.
Database opened.
SQL>