SQL*Plus: Release - Production on Thu Apr 15 17:00:06 2010

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Enter user-name:scott
Enter Password: tiger

ORA-01034:Oracle not avaliable
ORA-27101hared memory realm does not exist

and then i am trying like this
(my sid is PLSExtProc) set oracle_sid=PLSExtProc
conn system/manager as sysdba

then again i can get the error as ORA-12560: TNS: protocal adapter error

how can i solve these errors and how can i connect to my database ,please tell me the solution and mail to me ranga545ranga@gmail.com