Duckling not eating?
It may be an abdominal infection. Sometimes bacteria can get inside, via the spot that the 'umbilical cord' comes out. I happens during the hatch. I would guess that that is what may be wrong here. The condition is called omphalitis. Have a look at its tummy where the cord came out (where the yolk got absorbed). Does it look sore, swollen or rough in that spot?
Thanks for the extra info. I'd say that omphalitis is very likely. A picture would be helpful though.
If it is omphalitis, then I'm very sorry to say that the outlook is not good. The only option would be to take your duckling to a vet that knows about birds and that can give the appropriate antibiotics. Even with antibiotics, that type of infection is one that is very hard to cure, and so it is possible that your duckling will not make it. I'm really very very sorry about that. Is a vet trip a possibility?
Ok, just read your later post about the belly not being warm and looking normal. A picture of her tummy (in focus) would be very useful.
Another possibility is a blockage of the intestine cause by eating something she shouldn't. What kind of bedding have you been using in her brooder? Have a look at her vent - does it look red or swollen or blocked up with poop at all?