Oracle Database Discussions

Hello all,
The database was working fine (11g) but 2 days ago suddenly I got this error:
An error was encountered performing the requested operation:

ORA-01034: ORACLE not available
ORA-27101: shared memory realm does not exist
01034. 00000 - "ORACLE not available"
*Cause: Oracle was not started up. Possible causes include the following
- The SGA requires more space than was allocated for it.
- The operating-system variable pointing to the instance is
improperly defined.
*Action: Refer to accompanying messages for possible causes and correct
the problem mentioned in the other messages.
If Oracle has been initialized, then on some operating systems,
verify that Oracle was linked correctly. See the platform
specific Oracle documentation.
Vendor code 1034

Unfortunately I am not familiar with the oracle very much I just developing some application using the data storing in oracle, and the server responsible will not be around in weeks!
I looked up the forum and found no proper answer, I shouted down the server and start it up again and used these solution: set oracle_sid = grms
sqlplus /nolog
conn system/manager as sysdba
startup

but it did not worked either.

Please guide me how this problem can be solved.

Thank you so much