For people age 40 and older with neck pain for less than two weeks that does not worsen with the application of pressure, the most common causes are
muscle spasm or strain
osteoarthritis - less likely if under age 30, no prior arthritis or injury in the neck.
Neck arthritis as a cause of neck pain is relatively uncommon in persons under the age of 40, however.
Fortunately, there is usually a good response to treatment in these conditions - that is, the prognosis is generally good.