oracle sqlplus登陆问题

用oracle用户登陆aix主机后
>sqlplusEnter user-name: sys
Enter password:
ERROR:
ORA-01034: ORACLE not available
ORA-27101: shared memory realm does not exist
IBM AIX RISC System/6000 Error: 2: No such file or directory请各位指点以上错误是何原因，我用pc机的pl/sql或者sqlplus登陆都没有问题，麻烦各位大侠指点！