ORA-03113: end-of-file on communication channel

Dear All;

Release 11.1.0.7.0 - Production on Tue May 14 13:39:17 2013

SYS@BAK AS SYSDBA> startup
ORACLE instance started.

Total System Global Area 1071333376 bytes
Fixed Size                  1318172 bytes
Variable Size             440402660 bytes
Database Buffers          616562688 bytes
Redo Buffers               13049856 bytes
Database mounted.
ORA-03113: end-of-file on communication channel
Process ID: 9781
Session ID: 400 Serial number: 5

SYS@BAK AS SYSDBA>

alert_BAK.log

Setting recovery target incarnation to 2
Successful mount of redo thread 1, with mount id 1251705112
Database mounted in Exclusive Mode
Lost write protection disabled
Completed: ALTER DATABASE   MOUNT
Tue May 14 13:38:36 2013
ALTER DATABASE OPEN
Beginning crash recovery of 1 threads
 parallel recovery started with 8 processes
Started redo scan
Completed redo scan