How To Recover a Dropped Index? - Oracle DBA FAQ - Managing Oracle Table Indexes

If you have the recycle bin feature turned on, dropped indexes are stored in the recycle bin. But it seems to be command to restore a dropped index out of the recycle bin. FLASHBACK INDEX is not a valid statement. See the following script:

ALTER SESSION SET recyclebin = on;
Statement processed.

CREATE TABLE student (id NUMBER(5) PRIMARY KEY,
  first_name VARCHAR(80) NOT NULL,
  last_name VARCHAR(80) NOT NULL,
  birth_date DATE NOT NULL,
  social_number VARCHAR(80) UNIQUE NOT NULL);
Table created.

CREATE INDEX student_birth_date ON student(birth_date);
Index created.

DROP INDEX STUDENT_BIRTH_DATE;
Index dropped.

SELECT object_name, original_name, type, droptime
  FROM recyclebin;
OBJECT_NAME             ORIGINAL_NAME      TYPE  DROPTIME
----------------------- ------------------ ----- ----------
BIN$1LlsjTxERKq+C7A==$0 STUDENT_BIRTH_DATE INDEX 2006-04-01

FLASHBACK INDEX student_birth_date TO BEFORE DROP;
ORA-00905: missing keyword