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
minor injury (as may occur with sports or a minor fall)
osteoarthritis
disc disease - often associated with radiating pain, numbness or tingling into one arm.
Muscle strain or spasm is possible (mostly because it's so common) but these tend to be painful with the application of pressure.
Fortunately, there is usually a good response to treatment in these conditions - that is, the prognosis is generally good.