Chinchilla unexpectedly passed away?

I have 3 chins. One three year old who is in a cage alone (smokey), and two four year olds in a cage together (whitey and diesel). I got smokey from a friend and I got whitey and diesel on Craigslist. The owners did not take good care of them. When I got whitey and diesel, they told me they almost died over the summer because they didn't know they had to be kept at a certain temperature and they also informed me they were only ever taken out of the cage for dust. However I took excellent care of my chinchillas, and I woke up to diesel not moving.. He passed away sometime between 5am-11am. He was fine last night, active as usual. They had food and water. The room was the right temperature. He didn't show any signs of illness, and whitey seems perfectly fine just a little distressed over the loss of his buddy. He is still active and eating and drinking. I just want to know what could have possibly happened to diesel? He is only 4. And if he is dead, why isn't whitey? If he had an illness surely whitey would have caught it as well. Please any information will help, I am devastated and want to make sure my other babies are safe. There was no plastic in the cage either. Thank you