J-1 U.S. Visa - Sealed envelop ?

the sealed envelope should contain form DS 2019. Did you get it back at the interview? It might or might not have been in a sealed envelope, but you will need form DS 2019. The SEVIS DS-2019 form acknowledges that your sponsor has invited you to visit the institution, has authorized your visit, and supports your application for J-1 status. It also contains start and end date. See whether you still have this. If not you will have to contact the embassy ASAP.