why is my 5 week brown leghorn dead?

[tsirkucp]

woke up this morning and 5 week old chicken was found dead. We have 8 chickens that we have been raising from 1 week old chicks. Have seemed perfectly healthy, feeding them organic chick starter with corn scratch. We took the heat lamp away about 1 week ago. They are in a box(with wood shavings in the bottom) in our house and it gets maybe down to 68 degrees at the lowest at night. There was no sign of trauma, was just laying on side in corner with eyes closed. Other chicks seem fine, poop looks normal. Any ideas, is this normal???