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 strain
minor injury (as may occur with sports or a minor fall).
osteoarthritis - less likely if under age 30, no prior arthritis or injury in the neck.
Fortunately, there is usually a good response to treatment in these conditions - that is, the prognosis is generally good.