I am so sorry to hear you lost the little one.
With chinchillas, the least little change from the norm can be a warning that something is wrong, long before it is evident that it is sick. Chinchillas are very much creatures of habit ... when that changes, there can be trouble brewing in the background.
Something as simple as drinking 20 % less water or eating 20% less food. OR the poops/feces being watery, having a covering of mucus, small and hard, or smaller in number, giving off a bad odor that is pungent ... it may sound crazy, but you can often tell more about what is wrong, or that something is going wrong, simply by the way the poop looks, smells or the number of 'little presents' it is or is not leaving.
As a general rule, when you finally realize a chin is sick because it looks sick, it's almost to late. In the wild, they had to hide an illness or injury or become easy prey. This trait is still with them today.
As Jamie said, please tell us everything you can about the chin for the last week ... even if you think it would not matter ... it may.
Jo Ann