This will probably sound funny, but...
You can tell alot about whats going on with chickens from their poop.
I wish I could give you an answer about what killed them, but there are alot of things out there that can kill them silently such as viruses and parasites and there are missing pieces to the puzzle.
When you get more chickens (not IF) try to be aware of their poop, there are some excellent links to articles that can help you diagnose problems.
dlhunicorn has an excellent page on poo-oligy.
As a side note: Cracked corn should be fed as a treat as it has little nutritional value, and will raise body temp. Best fed in winter to help them stay warm.
Sorry for your loss, don't give up.