If you saw ORA-01034 in the alert log, don't panic, it's not generated from the database that you are currently running.
Tue Oct 13 14:35:08 2008
Errors in file /oracle/admin/ORCL1/bdump/orcl1_arc1_3295892.trc:
ORA-01034: ORACLE not available
It's usually from one of other databases which is connected via database links or archive log destination (Log Transportation). In our case, the most suspicious database is the standby database, because the trace file in the above message shows the error was an archiving operation.
Now, you should check the trace file for more information. I think the remote database that occurred the problem was terminated unplannedly for some reasons.