What are the reason and causes of neck pain?

Lots of them. There's 3... 1. Stress. 2. Sleeping in an uncomfortable position. 3. Worry.

There are lots of them. Stress or poor posture, especially if you are slumped when using your computer/tablet/phone. Injury from being jerked. Even the wrong pillow can give you neck pain.

Stress, if you sleep wrong side and pillow is affect on neck.

If you are continuously watching  on computer, tablet , smart phone so it's bad for neck.

You might have pulled a muscle .

Neck pain can be caused from some sort of stress in the muscles, as you can see you have a giant triangular muscle on the shoulders and neck called the trapezius, so your neck pain can actually originate from your occipitals (base of your skull), or your shoulders. See if you could get a friend or partner to massage these areas or to use a used deodorant ball stick to rub over these areas to see where you have pain.  Then massage that area either with hands or the deodorant stick to release the tightness.

Some neck pain can come from degeneration in the cervical spine, but using massage will help to see if it is muscular related. Cover the neck (be careful when applying pressure), the shoulders (including around and on the shoulder blades), the pectorals and the jaw (use fingers as pressure in front of the ear area and just behind it). You can even see if there is tightness in the skull by lightly rubbing your knuckles over it (indian head massage technique).

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