This sounds like a very serious problem with the neurological function. With x-rays you know basic
skeletalSkeletal limb abnormalities function and formation, and you can see general soft tissue status.
The problem you
faceFace pain may be "soft tissue" in origin and therefore may be difficult if not impossible to diagnose with simple x-rays. Other testing will need to be done.
Neurological infection (say viral), tumor (spine or abdomen), blood
clotsBlood clots etc. and the list goes on - and as you can see may be very difficult to diagnose.
Please work with your veterinarian closely for a diagnosis. You may also want to see if you can find a veterinarian who is Board Certified in Internal Medicine (ACVIM) as they have advanced training in such diagnostics.
I agree with Dr. Humphries comments.
There are many possible causes as Dr. Humphries stated.
Since the x-rays are not showing the problem, I would ask your vet to run a complete set of blood (CBC and Chemistry Profile) and urine tests (urinalysis) so that you can evaluate the internal status of your dog.
Once you have this information, it should help to narrow down a diagnosis.
Because your dog is not eating, it is also important to maintain hydration levels and prevent dehydration.
I hope this is helpful.
Thank you,
Dr. Carol Osborne, DVM