If you lose another one in the next few days you can do a necropsy to see what's wrong. At this age, unless your losing a lot everyday, can only be a handfull of problems.
First open the bird up.... if lot's of fluid comes out the cavity (yellowish) than it was flip.... If this is the case keep an eye on the others and possibly restrict the feed. Best way to do this is at night and simply turn the lights off. If it's too cold to turn the lights off use a red bulb at night.
If no fluid.... go straight for the liver... When you stick your hand in it's the first thing on top just below the heart. Grab it and pull it out... if you see little pin marks on the liver or white spots it could very well be cocci... It could also be a handfull of other diseases as well and only a culture can verify which one. Your closest State University can run the test for under 30 bucks.
Jeff