Oracle Database Discussions

Hi there, I am new to Oracle database and have been working on a test installation of oracle 11g that my employer wants me to practise on. My oracle instance was working fine, and then I logged in and found the following error when I try to start the instance.

SQL> startup
ORACLE instance started.

Total System Global Area 1653518336 bytes
Fixed Size 2176288 bytes
Variable Size 1006635744 bytes
Database Buffers 637534208 bytes
Redo Buffers 7172096 bytes
ORA-03113: end-of-file on communication channel
Process ID: 5736
Session ID: 191 Serial number: 1

I get the same error if I try start with RMAN:

RMAN> startup mount

Oracle instance started
RMAN-00571: ===========================================================
RMAN-00569: =============== ERROR MESSAGE STACK FOLLOWS ===============
RMAN-00571: ===========================================================
RMAN-03002: failure of startup command at 03/07/2012 15:18:29
ORA-03113: end-of-file on communication channel
Process ID: 7256
Session ID: 192 Serial number: 5

In Enterprise Manager I can see the instance is down, the listener is up and in the section Agent Connection to Instance it says:

Status
Failed
Details ORA-12505: TNS:listener does not currently know of SID given in connect descriptor (DBD ERROR: OCIServerAttach)

Ok, so for both these errors ORA-12505 and ORA-03113 I can find plenty of information on the net, but I have not been able to make any progress and just feel like I am going around in circles, hence my post here. The last thing I did when the instance was working was switch to archive logging mode. Then the next day started getting these errors. Any help much appreciated... but at this point I am not sure what to do next!