ORA-01092: ORACLE INSTANCE TERMINATED. DISCONNECTION FORCED

###########################################################
ORA-01092: ORACLE INSTANCE TERMINATED. DISCONNECTION FORCED
###########################################################

SQL> startup
ORACLE instance started.

Total System Global Area 1068937216 bytes
Fixed Size                  2191112 bytes
Variable Size             426684664 bytes
Database Buffers          629145600 bytes
Redo Buffers               10915840 bytes
Database mounted.
ORA-01092: ORACLE instance terminated. Disconnection forced
Process ID: 14492
Session ID: 690 Serial number: 1

Solution:
---------

check for alertlog..

create the undo tablespace.

CREATE UNDO TABLESPACE "UNDOTS2" DATAFILE '+OAD' SIZE 512M REUSE AUTOEXTEND ON NEXT 100M MAXSIZE 2G, '+OAD' SIZE 512M REUSE AUTOEXTEND ON
NEXT 100M MAXSIZE 2G , '+OAD' SIZE 512M REUSE AUTOEXTEND ON NEXT 100M MAXSIZE 2G;