you need to investigate those deadlocks
problem is that , Database stop respone to application Server but still it is giving response to Toad and others things .this problem we faced two times in production database and also *Database Shutdown abort still * *not worked properly *and after restrarting database it;s start response to database setver
Problem in Alert Log file -
Mon Jul 10 13:12:24 2006 ORA-000060: Deadlock detected. More info in file d:\database\udump\vvvS001.TRC.