Muscle Strain: The sudden onset of neck pain radiating down the neck without a known mechanism of injury could be due to a muscle strain, which is a common condition. The fact that it started in the morning suggests possible strain from an unusual sleeping position or sudden movement.

Cervical Spondylosis: Given the patient's age, cervical spondylosis (wear and tear on the disks in the neck) could be a cause of the pain, especially if there's been a recent increase in activity or a change in posture.
Herniated Cervical Disk: A herniated disk in the cervical spine could cause pain on one side of the neck, radiating down, due to nerve compression.
Tension Headache: Although the pain is described as being on the side of the neck, tension headaches can sometimes present with neck pain and could be considered, especially if there are stressors or if the patient has a history of headaches.

Cervical Artery Dissection: This is a serious condition where there is a tear in one of the arteries in the neck, which can lead to a stroke. It can present with sudden neck pain and is crucial not to miss.
Myocardial Infarction (MI): Although less common, cardiac conditions like MI can sometimes present with referred pain to the neck, especially on the left side.
Pulmonary Embolism: In rare cases, a pulmonary embolism can cause referred pain to the neck, especially if the embolism is large and affects blood flow significantly.

Multiple Myeloma or Metastatic Disease: Although rare, cancers like multiple myeloma or metastatic disease to the spine can cause neck pain, especially if there's a history of cancer.
Infectious Diseases (e.g., Meningitis, Abscess): Infections such as meningitis or an abscess in the spinal cord or neck region can cause severe neck pain and stiffness, often accompanied by fever and other systemic symptoms.
Temporal Arteritis: This condition, more common in older adults, involves inflammation of the blood vessels and can cause severe headaches and neck pain, among other symptoms.