Just thought I would say sometimes they die due to genetics issues, sudden death can happen and does quite often, i have lost several from different matings at different stages in the lives.One batch died in a couple of weeks, all except 3 of 20. Another time I had a batch die at approx 6 months and was left with only 1.
I to was worried I had done something wrong but they had their own tubs, fresh dechlorinated water daily, plenty of food and a temp of 15 degrees max. They were eating fine and seemed very happy but in a matter of hours just started to perish 1 by 1.I went on to a site from a univercity that was doing genetic studies and it stated that they had also encountered the same thing sometimes hundreds would just die, they stated that it was down to a genetics issue.
If you do try again and it happens from the same pairing it might be an idea to find another mate.