PROBLEM:

While starting the database, got below error.

SQL> startup
ORACLE instance started.

Total System Global Area 1.2549E+10 bytes
Fixed Size 12155024 bytes
Variable Size 6744442736 bytes
Database Buffers 5771362304 bytes
Redo Buffers 21397504 bytes
Database mounted.
ORA-38760: This database instance failed to turn on flashback database.

SOLUTION:

1. Check the alert log:

Errors in file /oracle/app/oracle/diag/rdbms/db12cr2/DB12CR2/trace/DB12CR2_rvwr_2776.trc:
ORA-38701: Flashback database log 1 seq 1 thread 1: "/oracle/app/oracle/product/12.2.0/dbhome_1/dbs/arch/FRA/DB12CR2/flashback/o1_mf_djfp4fjd_.flb"
ORA-27037: unable to obtain file status
SVR4 Error: 2: No such file or directory
Additional information: 7
2017-05-01T09:43:24.075852+03:00
WARNING: Cannot open the flashback thread for this instance due to the above error.
WARNING: Flashback thread open failed - to resolve this, either correct the reported error or turn off database flashback.
2017-05-01T09:43:24.076337+03:00
Database mounted in Exclusive Mode
Lost write protection disabled
Using STANDBY_ARCHIVE_DEST parameter default value as USE_DB_RECOVERY_FILE_DEST
Completed: ALTER DATABASE   MOUNT

2. Check the flashback_mode of the database.

SQL> select name,flashback_on from v$database;
 
NAME      FLASHBACK_ON
--------- ------------------
DBATEST   YES ----- > FLASHBACK IS ENABLED...

From the alert log, it is clear that flashback mode is enabled in the database and someone has mistakenly deleted flashback log physically.

So, to fix this issue, the only solution is to disable the flashback mode and open the database.

SQL> select name,open_mode from v$database;

NAME      OPEN_MODE
--------- --------------------
DB12CR2   MOUNTED

SQL> alter database flashback off;

Database altered.

SQL> alter database open;

Database altered.