Why won't my dog eat?

It could because of an upset stomach.

 If your dog has eaten something they shouldn't have, the offending snack can cause gastrointestinal issues that may take some time to get over.

Even the most well-trained dog is tempted to gobble up trash, bugs, and other seemingly tasty objects.When this happens, the dog will probably experience some stomach pains and attempt to get rid of whatever they ate. They may start to vomit or eat grass. Dogs may also experience issues like diarrhea as a result. Either way, the feeling in their stomach will often make them avoid food until they're starting to feel better.

Text: https://www.dailydogstuff.com/dog-wont-eat/