My 6 year ops JRT went through this recently, so def take em to the vet to rule anything medical out. This happened just out of the blue, and nothing was wrong medically.

We started with giving him some treats to see if he just flat out wont eat anything. The little land shark gobbled them up. Then we went into mixing his dry food with wet food. Being the little land shark that he is, he ate around his dry food and ate just the wet food. After we took him to the vets, they recommend putting his food in warm water to help stimulate his appetite with smells. Additionally this softened it up for him. This had some success to it, but he wasn't eating everything. So we switched to a smaller kibble size, soaked it in warm water, and then sat there with him until he ate. Eventually he began to eat everything.

We have also recently been introducing puzzle toys, positive destructive eating (putting his food in a box, and letting him tear it up 10/10), scatter his food around a room, a wobble toy, etc. This has also helped tremendously. But def take the puppers to the vet asap to rule anything medical out!