If only it were that easy. You can't tell what killed a chicken by just looking at it unless it has, for example, a missing head. Then you might guess correctly that the chicken died from getting its head cut off.
When we want to know what killed a chicken, and it's not something obvious like a missing head, we send the dead chicken to an agricultural lab and request a necropsy. This is where a lab technician cuts the chicken open and takes out all of the organs and searches for abnormalities. Sometimes tissue samples are sent out for further tests to see if bacteria or viruses or a toxin are responsible.
If you want answers, refrigerate the body, and ask your grandpa if he can help you find a lab. You can start by calling your vet or university extension office and ask where the nearest lab is. Then call the lab and ask how much it will cost. It can range from free to $20 to $100.
We here on BYC can ask you a lot of questions and get clues that might help us make guesses as to what caused this chick to die, but the best way to get an accurate answer is to get a necropsy.
