Hi Sir or Madam,
This could be swollen lymph gland, abscessed tooth or possibly an salivary gland problem depending on where on the neck it is. Usually with a salivary gland problem you will also see excessive drooling.
Swollen lymph nodes can be due to many causes including infections, autoimmune diseases, fungal and bacterial infections and lymphoma (cancer). You can read about the lymphatic system in the dog here:
An abscessed tooth can sometimes cause lumps or swelling of the muzzle or jaw and upper neck. You will want to check your dog's mouth and look for any broken or discolored teeth. If you find any, buffered aspirin can be given to a dog with a dosage of up to 5 mg per pound every 12 hours for pain until your dog can be seen by your Vet. Keep in mind that a dog's body does not metabolize aspirin in the same way as a human and thus should not be given more than a day or two without contacting your Vet. The aspirin may need to clear your dog's system before other medications can be given, so keep that in mind if you decide to give aspirin and be sure and tell your vet when your dog is seen.
Check just under your dog's jawbone for a swelling and under your dog's tongue for a swelling. This is where some of your dog's salivary glands are located. If you find a swelling here, it's possible your dog has an accumulation of fluid near the salivary gland called a sialocele which is causing your dog to salivate more. This condition does require your dog to be seen as soon as possible. You can read about it here:
While your dog's lymph nodes can swell for a number of reasons such as infection, it could be lymphoma which is common in dogs. The first sign of this would be swollen lymph nodes. the only way to definitely diagnose lymphoma would be to have a biopsy done. Blood tests might help but a biopsy is the best way. If this is lymphoma, your vet might treat with prednisone or a more aggressive treatment might include chemotherapy. You can read more on this here:
Here is a site that discusses alternate reasons for swollen lymph nodes.
At a minimum, I would have bloodwork and a urinalyses done. X-rays would likely be done to look for any mass that might be present. An allergic reaction can also cause swelling. If you feel it might be an insect sting or bite, benadryl can be give at a dosage of 1mg per pound every 8 hours.
I hope this information is helpful to you.