ALert log show index is unusable

2013-07-29 08:15:21.300000 +05:30
Incremental checkpoint up to RBA [0x4f.10bae8.0], current log tail at RBA [0x4f.10cc43.0]
2013-07-29 08:33:39.191000 +05:30
Index MSC.MSC_COSTS_F_N1 or some [sub]partitions of the index have been marked unusable
Index MSC.MSC_COSTS_F_N2 or some [sub]partitions of the index have been marked unusable
Index MSC.MSC_COSTS_F_N3 or some [sub]partitions of the index have been marked unusable
2013-07-29 08:33:41.179000 +05:30
Index MSC.MSC_SUPPLIERS_F_N1 or some [sub]partitions of the index have been marked unusable
2013-07-29 08:33:43.171000 +05:30
Index MSC.MSC_DEMANDS_CUM_F_N1 or some [sub]partitions of the index have been marked unusable