An ORA-01034 error occurs when Oracle is not started. It can be caused by a problem in the System Global Area (SGA). Another common cause is the operating system variable pointing to the instance being improperly defined. If you shut the database down with "NORMAL" option and there are active sessions you may keep getting this message when you try and start the instance. If you are trying to login you may need to set the TWO_TASK variable.
How to fix it
If you get a ORA-01034 when you issue the login command (in sqlplus, imp, exp,..) without giving the DB name, check the value of the TWO_TASK environment parameter. This is the database to which a user connects by default. Maybe the TWO_TASK points to a database which is down or does not exists at all. Try export TWO_TASK=db.
Older versions of Oracle used to leave a lock file in the $ORACLE_HOME/dbs directory - like lkSIDNAME. The way to remove this file is to:
svrmgrl connect internal shutdown abort; startup