It is possible that your little turtle over heated. A temperature of about 94F can be lethal to an incubating egg, so it doesn't seem like a stretch to think that similarly high temperatures could be equally fatal to a hatchling.

Ideally an indoor habitat should have a temperature gradient from about 70F or so at one end to 85F at the other so that a turtle can move around, cool off, warm up and generally regulate its body temperature on its own.