Why does this happen?
A long day of sitting or standing in poor posture causes your posterior neck and thoracic muscles to be stretched, and your anterior muscles to be shortened. This suboptimal environment causes your muscles to work harder to hold you upright.
Even though this is a sub conscious effort, our muscles will eventually fatigue. As metabolites accumulate, the nerve ending are stimulated, and cause the classic achy sensation in your neck and back.
What can be done?
Lay down with a heat pack over the area, allowing muscles to relax back into a neutral position. Take some deep breathes to calm down your sympathetic nervous system, which can amplify pain.
The common culprits are concentrically shortened levator scapulae, and pectorials
Hold for 20 seconds, repeat twice
Thoracic cat camels for 30 secs, repeat twice
Scapular squeezes hold 5, repeat 10 times