I just found my little Serama dead! It was fine last night- and I even made a comment that it was getting little wing feathers! I have no idea why it would live a week, apparently healthy and then be dead- with no symptoms. The other chicks are fine- the two pullets I bought to be its friends, and the BG that hatched out on Saturday.
I understand, that happened to me once. I am sorry for your loss. I hope you have lots of other chickens to take care of so it won't be so bad.
Take care and my thoughts are with you!