That’s a nightmare, but try not to worry too much. There are stories like this on here every year, it might be worthwhile searching back the forum to this time last year to see how people resolved it.
As far as I can remember, everyone I saw who had Royal Mail lose the ticket ended up getting into the festival.
It just takes a bit of persistence with Royal Mail, see tickets etc.
If the worst comes to the worst, you can still always just rock up to the festival site and go to the box office - it might take a few hours, but I’m sure you’d be able to get in.
Best of luck!!