Encountered this issue when I came into the office this morning.

The verbiage indicates that Oracle is starting up (or down), but waiting a bit doesn’t cause the error to go away. Time for Google-ing.

$ sqlplus sys/xxxxxxx as sysdba

Replace the ‘xxxxxxx’ part with the actual password, of course. If the ’sqlplus’ command is not recognized, it’s probably a problem with your envvars, but that’s a whole can of worms altogether. But I digress. Let’s see whether the db can be mounted:

SQL> alter database mount;
ERROR at line 1:
ORA-01100: database already mounted

OK, that’s a start. How about open-ing?

SQL> alter database open;
ERROR at line 1:
ORA-01113: file 1 needs media recovery

OK let’s do recovery:

ORA-01110: data file 1: '/usr/lib/oracle/xe/oradata/XE/system.dbf'
SQL> recover datafile '/usr/lib/oracle/xe/oradata/XE/system.dbf'
Media recovery complete.
SQL> alter database open;
Database altered.
SQL> quit
Disconnected from Oracle Database 10g Express Edition Release 10.2.0.1.0 - Production

Hope this helps somebody.