You need to apply for a new Bus & Tram Discount photocard at a post office in London.
If you are on Universal Credit or Job Seekers Allowance, you will need to apply for a replacement Bus & Tram Discount photocard at your local JobCentre Plus office.
You will need to get the Bus & Tram Discount re-set on your Oyster card once you get your replacement photocard. You can do this at Tube, London Overground, TfL Rail and some National Rail stations, as well as Oyster Ticket Stops and Visitor Centres.
If you've lost your Oyster card as well, you'll need to get a new one before you can re-set the discount.