SAP R/3 got stopped with ORA-01034 error

Hi all,

My SAP R/3 4.7,got stopped, and i am getting ORA-01034: Oracle not available

ORA-27101: Shared memory relam does not exists, what are the steps i have to follow to make my R/3, with which user ID i have to log-in to oracle and what are the things i have to check to make my SAP up? I am unable to connect to my ORACLE instance.