You may be facing oracle is not accessible or not available from application or console. First and foremost try is to shutdown database and restart it.
When you go for sqlplus startup
SQL*Plus: Release 10.2.0.1.0
Copyright (c) 1982, 2005, Oracle. All rights reserved.
Enter user-name: sys as sysdba
Sql> Shutdown Immediate
ORACLE instance started.
Total System Global Area 189675120 bytes
Fixed Size 1289586 bytes
Variable Size 69808522 bytes
Database Buffers 87996216 bytes
Redo Buffers 6139823 bytes
ORA-00600: internal error code, arguments: [kcratr1_lastbwr], , , , ,
, , 
ORA-00600: internal error code, arguments: [kcratr1_lastbwr], , , , , , ,  due to sudden power failure.
when try to startup Oracle 10g database. it shows that database mounted and then followed by this error.
This problem can be solved by using restoring the database with the help of below commands
1. Shutdown the oracle instance
Sql> shutdown immediate
2. Start the database with MOUNT option
Sql> Startup Mount
3. Recovery the database using Recover Command
Sql> Recover Database
4. Open the database
Sql> Alter Database Open
Restart oracle services if possible and needed. These steps will resolve ORA-00600 problem.
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