There are many, many possible causes for this. So, let me go through a few of the most possible.
First, how long has it been? If everything else seems to be “normal” with your pet, it’s okay to go up to 24 hours without eating. It’s possible he has the flu.
Of course, look for missing toys. It’s possible one has been ingested, and now has an intestinal obstruction.
Next, check his temperature. (Normal is 101-102.5.) If you don’t have a thermometer to use, check her ears. They should be warm close to the head, cool at the tip. If they’re warm all the way to the tip, it’s possible your little guy has fever. If so, get your pet to the vet. (If your pet is a puppy who isn’t fully vaccinated, you could be dealing with Parvo. RUN to your vet’s office!)
If there is any vomiting or diarrhea, lift up the skin on the back of his/her neck – it should instantly go back down. If it doesn’t, they could be dehydrated. Call your vet and tell them the situation.
Of course, your dog could just be finicky – he’s trying to tell you something about that yucky kibble someone keeps sticking in his dish! (I think he’s saying, GO TO www.dinnerpawsible.com and whip me up something worth eating!)