Oracle Solution for Error ora-01502

Solution for Oracle Error ORA-01502

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

What triggered the Error:

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

What should we do to fix it:

DROP the specified index, or REBUILD the specified index, or REBUILD the unusable index partition