Yes, I’m not exactly sure what you are asking. That’s not your fault at all, I’m glad you asked the question. It got the conversation started. If you knew what information we needed you probably would not be asking the question to start with. We all have to start somewhere in our experience and knowledge.
How old are they? Where did you get them, in the mail, from a feed store, did you hatch them yourself, or maybe some other method? How as the chick acting before it died, was it standing around peeping, fluffed up, and lethargic or was it acting normal like the others? Are the others standing around lethargic? Tell us a bit about where they are living too, in a brooder or something else and what does that brooder look like and where is it? Were there any injuries on it? Do you have older chickens with it?
Especially if it is less than week old it could be what we call failure to thrive. Some are just hatched with birth defects, often internal where you can’t see them, and are not meant to make it. Or maybe a heart defect that caused a heart attack. If it is only one and the others are thriving it’s probably not something you did. But a little more info might help us give better answers.