I'm sorry this chick died. It's sad when this happens and leaves a helpless feeling as well as sense of loss.
This week I had a new chick take a sudden turn south. She was right at the one week mark and being raised by a broody hen. The first thing I noticed was she was slower than the other chick and was peeping non-stop. She couldn't seem to get warm and was trying to be under the hen all the time.
When a chick suddenly takes sick after appearing normal, I suspect constipation or crop problems. Both can kill a chick very quickly. I took this chick indoors and got out a tub of coconut oil. Taking a few small chunks of the solid oil, the solid form being harder to aspirate, I pried open the chick's beak and placed it on her tongue. The chick immediately swallows when you do this. I probably got about half a teaspoon of oil down her.
Then I returned her to her broody mama. Sometime during the night, the chick passed a turd that was of a size you'd have thought there was a much larger chick in the nest. She's been fine ever since.
Whether or not a sick chick is constipated, this oil treatment can't hurt, and it often can restore a sick chick back to normalcy.