ORA-01034
ORA-01034 means that the command you want to perform cannot be done in a stop, idle or shutdown database. If you're surprised by the error, maybe you should check the database status immediately. It maybe someone or something turn it off without notification.
Connect to Local Database
Let's see a case. Tried to connect to the local database.
[oracle@test ~]$ sqlplus system
...
Enter password:
ERROR:
ORA-01034: ORACLE not available
ORA-27101: shared memory realm does not exist
Linux-x86_64 Error: 2: No such file or directory
Additional information: 4376
Additional information: -1649756641
Process ID: 0
Session ID: 0 Serial number: 0
Because the database is stop there's no way to do authentication.
Let's see another case.
[oracle@test ~]$ sqlplus / as sysdba
...
SQL> alter database begin backup;
alter database begin backup
*
ERROR at line 1:
ORA-01034: ORACLE not available
Process ID: 0
Session ID: 0 Serial number: 0
Although we can connect to the database by OS authentication, we cannot perform any database operation.
The solution is to startup the database.
Data Guard
If you saw ORA-01034 in the alert log, don't panic, it's not generated from the database that you are currently running. It's usually from the standby Database.
...
Tue Oct 13 14:35:08 2008
Errors in file /oracle/admin/ORCL1/bdump/orcl1_arc1_3295892.trc:
ORA-01034: ORACLE not available
In such case, the primary database tried to transport redo log to the standby database, but the standby database is stop for some reason. You need to check what's going on.