Loss of appetite and refusal to eat in any dog is always a sign that something is wrong. It could be anything, from upset tummy to severe pain. You don't know, only the vet can tell what's going on.
Is your puppy otherwise energetic and OK?? Or sleeping a lot/not playing/shivering or shaking/throwing up/diarrhea??
Yes, in order to help really we need more info.
Anyway, even WITH the info, we can only give you some support. We can't diagnose your dog or really help, practically. You need to take him to the vet.
Could be a sugar problem maybe diabetes, I agree he should be checked by the vet, what do you feed him ,you say he doesnt eat ..
You do not say if a he /she.You know you will have to take your pup to the Vet.
It could be many things.Remember they need to be loved and cared for also played with and hopefully live inside with you.It could also be the dreaded Parvo virus.
I am saying this as you give no details at all just out of the blue, like so many also we never here from you again.
This really worries me as you could be anyone YOU MUST put more detail Daniel your Pup deserves that much.We could then help you....so fill us in a bit more please