Oracle Database Discussions

Hello everybody,
I'm using Oracle 9i and I'm getting below error when I execute a anonymous block which runs through the cursor and updates the table.

ORA-01502: index 'ASAP.PK_STATUS_NO' or partition of such index is in unusable state

select query on the same table is executing fine but seems that update is getting troubled. And the status number column is declared to be primary key

Can anyone tell me what and where the problem might be ???

regards,
Rossy