Does she have a chicken buddy that doesn't bully her? I would offer her a soft boiled egg and see what happens. Often, if a chicken refuses to eat, being with another friendly chicken will inspire it to eat when the other chicken attacks the food.
I don't see how this chicken could have a bacterial infection just being in a tractor on grass. If she's been deprived of food, she may have reached the tipping point today where she no longer has the strength to go on or even to eat.
If she refuses to eat, you may need to try to syringe the food into her beak, but tube feeding would be safer and far more efficient. I got a feeding tube and a feeding syringe from my vet for just $3. It's easy to slide the tube into the right side of the throat and into the crop.
I would urge you to take a poop sample to your vet and ask for a fecal float test for worms and coccidia. Coccidiosis is a possibility. She may have an overload of coccidia due to her weakened condition from starvation. Cocciciosis can make a chicken suddenly very sick.