Like the others said - sometimes it's a problem in the brain - would help to get a vet assessment. Remember to warn the vet the dog is now "snappy".
What exactly do you do when the dog snaps - because this may be encouraging it. Eg scolding, and firm taps on the dog's nose are likely to encourage snapping. There could have been some superficial injury that started the snapping, and the attempted fix may have made the behaviour worse. What exactly triggers the snapping? Just it just happen randomly (probably a brain thing) or only if another person or dog approaches or only when there is food around (maybe behavioural) or what?
Some things can be fixed by training, some by meds and some can't eg terminal brain cancer. If it is disease thing - then please ask your vet to send the details of the dog, its breeding and the disease to the LIDA database.