sounds more like guppy fry now one has died. is your male to female ratio ok? my guppy died after giving birth. she seemed to forget how to swim and just sat on the bottom till she died.
I say platy - I have guppies and platties and platy fry tend to be shorter and stockier and show color right away where guppy fry are longer, thinner, and take longer to show color. A better, up close, photo would help though.