Oracle how to make primary key index usable from unusable?
01502, 00000, "index '%s.%s' or partition of such index is in unusable state"
  
  MERGE: 1489 RENUMBERED TO 1502
  
  Cause: An attempt has been made to access an index or index partition
          that has been marked unusable by a direct load or by a DDL
          operation.
  
  Action: DROP the specified index, or REBUILD the specified index, or
          REBUILD the unusable index partition.


Now, you have to rebuild index(es) according to their type, Global or Local.


  You can rebuild global index partitions in two ways:
  Rebuild each partition by issuing the ALTER INDEX REBUILD PARTITION statement (you can run the rebuilds concurrently).
  
  Drop the entire global index and re-create it. This method is more efficient because the table is scanned only one time.
  
  Rebuild local indexes using either ALTER INDEX or ALTER TABLE as follows:
  
  ALTER INDEX REBUILD PARTITION/SUBPARTITION
  
  This statement rebuilds an index partition or subpartition unconditionally.
  
  ALTER TABLE MODIFY PARTITION/SUBPARTITION REBUILD UNUSABLE LOCAL INDEXES
  
  This statement finds all of the unusable indexes for the given table partition or subpartition and rebuilds them. It only rebuilds an index partition if it has been marked UNUSABLE.


Documentation:About Rebuilding Index Partitions