If you can, take the receipt to the ticket office where you bought the original, say you need it reissuing because a conductor confiscated it and took your details claiming it was invalid because it was no longer readable, and ask if they can reissue it. (If you do this in office hours, they might be willing to phone up internally and try to found out what happened.) If they try to charge you a fee for a lost railcard, you might make a pragmatic decision to pay that and then attempt to claim it back later. (If they ask you to sign anything, be sure to note down the correct circumstances. So don't sign anything saying it was 'lost' - cross that out and write instead that the conductor on the X train from A to B on date Y confiscated it claiming the date was unreadable which you dispute.)