ORA-01034 : ORACLE not available ORA-27101: Shared Memory Realm Does Not Exist

ORA-01034 : ORACLE not available
ORA-27101 : shared memory realm does not exist 

Mainly database could be down. check the status using ps.

ps -ef | grep pmon


Check alert log for more details for the reason of shutdown.

$sqlplus

SQL*Plus: Release 12.1.0.2.0 Production on Tue Nov 19 18:00:26 2019

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

Last Successful login time: Tue Nov 19 2019 17:04:35 +05:30

Enter user-name: devdb12
Enter password:
ERROR:
ORA-01034: ORACLE not available
ORA-27101:  shared memory realm does not exist
Process ID: 0
Session ID: 0 Serial number: 0

To resolve this issue,  set the ORACLE_SID and ORACLE_HOME and startup database using the sys as sysdba.

export ORACLE_SID = DEVNDPI
export ORACLE_HOME = /u01/app/oracle/product/12.1.0
sqlplus / as sysdba
SQL*Plus: Release 12.1.0.2.0 Production on Tue Nov 19 18:02:53 2019

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

Last Successful login time: Tue Nov 19 2019 17:06:12 +05:30

Connected to an idle instance.
SQL> startup
ORACLE instance started.

Total System Global Area  535662592 bytes
Fixed Size                  1375792 bytes
Variable Size             331350480 bytes
Database Buffers          197132288 bytes
Redo Buffers                5804032 bytes
Database mounted.
Database opened.