Shared Memory Realm does not exist

ERROR:

ORA-01034: ORACLE not available

ORA-27101: shared memory realm does not exist

Solution :

check ORACLE_SID has value if not exist then set ORACLE_SID

For Windows Environment

set ORACLE_SID=orcl 
echo %ORACLE_SID% 
orcl 

For Linux Environment

$ export ORACLE_SID=orcl

Now You need to Manually startup the database

sqlplus "sys/password as sysdba" 
Connected to an idle instance.
SQL> startup;
ORACLE instance started.
Total System Global Area  574619648 bytes
Fixed Size                  1250236 bytes
Variable Size             192941124 bytes
Database Buffers          377487360 bytes
Redo Buffers                2940928 bytes
Database mounted.
Database opened.
SQL> exit