ORA-03113 when starting up an Oracle 11g database
From 
ParnassusData:


  This error indicates that a mismatch has been detected between redo records and rollback (undo) records. 
  
  ARGUMENTS:
  
  Arg [a] - Maximum Undo record number in Undo block 
  
  Arg [b]  -  Undo record number from Redo block
  
  Since we are adding a new undo record to our undo block, we would expect that the new record number is equal to the maximum record number in the undo block plus one. Before Oracle can add a new undo record to the undo block it validates that this is correct. If this validation fails, then an ORA-600 [4194] will be triggered.
  
  Oracle is unable to roll back an uncommitted transaction in its transaction table. You probably have corruption in either the rollback/undo segment or corruption in the object in which the rollback/undo segment is trying to apply the undo record.
  
  Refer to Note: 39283.1 for more details on the description of this error.


Oracle Note  39283.1 says more or less the same.

Here is a link where somebody solved the problem by creating a new  undo tablespace: SAP, Databases and other IT stuff

Here are the statements:

-- Creation of a new Undo tablespace:
CREATE UNDO TABLESPACE undotbs
DATAFILE 'c:\oracle\10201\dev\undotbs1.dbf'
SIZE 300M AUTOEXTEND ON NEXT 20M
MAXSIZE 800M;

-- Change the default undo tablespace to the new one
ALTER SYSTEM SET UNDO_TABLESPACE = undotbs SCOPE=BOTH;

-- Drop the problematic undo tablespace
DROP TABLESPACE oldundotbs;




To create a tablespace  the database must be open.

I don't know how to reproduce this error so I cannot test how to repair the system. I also think I won't get any feedback from the OP. But nevertheless let's think about, how we can proceed.


The best is to open a service request at Oracle Support.
One possibility is to restore the undo tablespace. But maybe the error will occur again. 
Then a point in time restore of the whole database will be an option. But this means loss of data.
We will try to repair the system by creating a new undo tablespace an dropping the old one as described above.


To repair the system we unset the UNDO_TABLESPACE parameter. Then the SYSTEM tablespace becomes the default tablespace and I assume the database can be opened now.

startup nomount
alter system reset undo_tablespace scope=spfile;
shutdown
startup


Now we can proceed as above and create and set a new undo tablespace an drop the problem tablespace as described above.