Zipped then died - Why?

[whudson]

I have one happy healthy chick.

An hour ago, the second had zipped about 85% around, so I decided to check again, and now he is dead.

What could possible causes be?

 

[americana-chick]

did you open the incubator?

 

[whudson]

No, Incubator hasn't been opened.
I have a plexi-glass top, and can see clearly that he isn't breathing.

This is my first try, and I'm glad to get the one, but I really didn't want him to be alone.
I only have 3 other eggs, and they haven't even pipped.

They were all fine when I candled Tuesday morning.