Oh no, that's unexpected?
Don't think you did anything wrong, sudden death at this young age would probably point to some kind of heart problem? And she was quite a bit overdue. You could ask the vet to open her up to see if that was the case, or anything else internally not being as it should be. If that#s the case you wouldn't want to repeat the mating with that specific sire.
RE her milk: I have no idea how easíly she would accept another kid to suckle, but you could try to suppplementary feed (by bottle or tube) another kid (triplet) and once her milk comes out at the end of that kid she may accept it. In the early stages it would probably need two of you to hold the nanny(one person) and latch on a kid (second person, until kid is confident, when only one person needed to hold the nanny). Tying the nanny up might help as well. BUT I have never had to do this. The nanny may try to sit down.
If she lets you milk her I would continue to milk and probably bottle feed any kids that need it.
I am so sorry, losing a kid this suddenly is just devastating.