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

Make sure that your ORACLE_HOME and ORACLE_SID are correct To see the current values in windows, at the command prompt type

echo %ORACLE_HOME%

Then

echo %ORACLE_SID%

If the values are not your current oracle home and SID you need to correct them. This can be done in Windows environment variables.

or

Open command prompt and execute the below commands:

cd $ORACLE_HOME/dbs or $ORACLE_HOME/database

orapwd file=PWDorcl.ora password=oracle

set oracle_sid=DATABASE NAME
sqlplus /nolog
conn sys/sys as sysdba
shutdown abort
startup