1. unusable a index:

    alter index GROUPS_PK unusable;

  1. find all unusable indexs:

    select * from user_indexes where status=’UNUSABLE’;

  1. rebuild a unusable index:

    alter index table_owner.index_name rebuild online nologging;

  1. rebuild all unusable indexs:
declare 
   cursor cur is select 'alter index '||table_owner || '.' ||index_name ||' rebuild online nologging' as sql_string from user_indexes where status='UNUSABLE';
begin 
  for item in cur loop
    EXECUTE IMMEDIATE item.sql_string;
  end loop;
end;