Cause
The database name in the control file does not match your database name.
Action
Either find the correct control file or change your database name.
This will be the error message
SQL>STARTUP ORACLE instance started. Total System Global Area 612368384 bytes Fixed Size 1292036 bytes Variable Size 239077628 bytes Database Buffers 364904448 bytes Redo Buffers 7094272 bytes ORA-01103: database name 'DB01' in control file is not 'DBDG'
Fix it :
1- Stop the database
SQL>shutdown immediate; ORA-01507: database not mounted ORACLE instance shut down.
2- Start you database in nomount mode and alter the DB_NAME parameter.
SQL>STARTUP NOMOUNT; ORACLE instance started. Total System Global Area 612368384 bytes Fixed Size 1292036 bytes Variable Size 243271932 bytes Database Buffers 360710144 bytes Redo Buffers 7094272 bytes SQL> ALTER SYSTEM SET DB_NAME=DB01 SCOPE=SPFILE; System altered.
3- Start up your database
SQL> SHUT IMMEDIATE; ORA-01507: database not mounted ORACLE instance shut down. SQL>STARTUP ORACLE instance started. Total System Global Area 612368384 bytes Fixed Size 1292036 bytes Variable Size 247466236 bytes Database Buffers 356515840 bytes Redo Buffers 7094272 bytes Database mounted. Database opened.