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