Today I encountered this ORA- error when trying to update a table:

ORA-29861: domain index is marked LOADING/FAILED/UNUSABLE

According to this post, I resolved the error by rebuild the "FAILED" index after identifying which index was corrupt: 

1 select index_name,index_type,status,domidx_status,domidx_opstatus from user_indexes where index_type = 'DOMAIN' and (domidx_status <> 'VALID' or domidx_opstatus <> 'VALID');

2 alter index &indexname rebuild;