Sometimes simplest of the errors get you bad….and this was one such time.
I was trying to connect to my Oracle XE DB using SQL*Plus but kept getting this errror

connect hr/************
ERROR:
ORA-01034: ORACLE not available
ORA-27101: shared memory realm does not exist

Turns out I did not start the DB instance.
Start -> All Programs -> Oracle Database 10g Express Edition -> Start Database