Unusable indexes

Oracle indexes can go into a UNUSABLE state after maintenance operation on the table or if the index is marked as 'unusable' with an ALTER INDEX command. A direct path load against a table or partition will also leave its indexes unusable.
Queries and other operations against a table with unusable indexes will generate errors:

ORA-01502: index ‘string.string’ or partition of such index is in unusable state

Detecting

The following SQL commands can be used to detect unusable indexes:
Indexes:

SELECT owner, index_name, tablespace_name
FROM   dba_indexes
WHERE  status = 'UNUSABLE';

Index partitions:

SELECT index_owner, index_name, partition_name, tablespace_name
FROM   dba_ind_PARTITIONS
WHERE  status = 'UNUSABLE';

Index subpartitions:

SELECT index_owner, index_name, partition_name, subpartition_name, tablespace_name
FROM   dba_ind_SUBPARTITIONS
WHERE  status = 'UNUSABLE';

Fixing

The following SQL will print out a list of alter commands that can be executed to fix unusable indexes:
Indexes:

SELECT 'alter index '||owner||'.'||index_name||' rebuild tablespace '||tablespace_name ||';' sql_to_rebuild_index
FROM   dba_indexes
WHERE  status = 'UNUSABLE';

Index partitions:

SELECT 'alter index '||index_owner||'.'||index_name ||' rebuild partition '||PARTITION_NAME||' TABLESPACE '||tablespace_name ||';' sql_to_rebuild_index
FROM   dba_ind_partitions
WHERE  status = 'UNUSABLE';

Index subpartitions:

SELECT 'alter index '||index_owner||'.'||index_name ||' rebuild subpartition '||SUBPARTITION_NAME||' TABLESPACE '||tablespace_name ||';' sql_to_rebuild_index
FROM   dba_ind_subpartitions
WHERE  status = 'UNUSABLE';

or if you prefer via single PLSQL anonymous block:

set serveroutput on size unlimited

BEGIN
 FOR x IN
 (
  SELECT 'ALTER INDEX '||OWNER||'.'||INDEX_NAME||' REBUILD ONLINE PARALLEL' comm
  FROM    dba_indexes
  WHERE   status = 'UNUSABLE'
  UNION ALL
  SELECT 'ALTER INDEX '||index_owner||'.'||index_name||' REBUILD PARTITION '||partition_name||' ONLINE PARALLEL'
  FROM    dba_ind_PARTITIONS
  WHERE   status = 'UNUSABLE'
  UNION ALL
  SELECT 'ALTER INDEX '||index_owner||'.'||index_name||' REBUILD SUBPARTITION '||subpartition_name||' ONLINE PARALLEL'
  FROM    dba_ind_SUBPARTITIONS
  WHERE   status = 'UNUSABLE'
 )
 LOOP
  dbms_output.put_line(x.comm);
  EXECUTE immediate x.comm;
 END LOOP;
END;
/