Why did this fawn die? Ideas?

In addition to my struggles with my own fawns (see other post) I got a shock first thing this morning, finding a 4 week old doe fawn that I was raising for the previous owner of my adults dead in the fawn pen. I had gotten her 2 weeks ago, caught off of her mom, after three days she quit being scared and had become very sweet. Throughout this past week I took her outside every afternoon, bringing her in at night until noon following day. She never took her milk from the bottle but from a dish, ate fruit, some grain, hay and greens in the pen (planted in foodplot mix, buckwheat, clover). Due to beautiful weather and the problems with the little does I left her outside over night for the first time. This pen is 20 yards outside my kitchenwindow, her bedding corner draped with tarp on outside, roof on top of kennel, with a haypile for bedding. Last feeding was right before dark around 9 p.m. last night. Drank the usual amount of milk, her bowel movements had been just excellent all along, big girl, ate also some of her canned mandarin slices. This  morning she laid stretched out in her bedding corner dead. No visible injury, some of the mandarin slices left. I am VERY distressed over WHAT HAPPENED? Was she scared to be out there for the first time, by herself? I had heard coyotes out back around 11 last night, would that send her into cardiac arrest? I don't think I had neighbor's dogs visit or my watchdog outside would have barked and she was friends with my son's Weimaraner and our outside dog. Does anybody have any clue as to what might have caused her death? I don't feel comfortable to leave the next fawn out there anymore.

Broken Neck?  Did she hit fence?

I wondered about it, but she was stiff and I couldn't detect anything feeling around.

my guess mandarin slices to acidic maybe just a guess T&S