HINT: Please REINDEX it.

Hi,
    When i start the postgres DB , it starts properly. but when i am
trying to connect to it . it shows a pop up with the error message
"Error connecting to the server: FATAL:  index "pg_class_oid_index"
contains unexpected zero page at block 0
HINT:  Please REINDEX it."
i tried starting the database using --single --P option , but says it
cannot be started with user as admin.

Any pointers on how to solve this problem?

Thanks in advance,
nsalam