It generally happens in case of prolonged sleeping posture specially side turn.. It leads to sapasm of trapezius /sternocleidomastoid muscle. Pain in moving the neck specially left/right side rotation or side bending.
Next Steps
Go to a physiotherapist.. Ultrasonic therapy +IFT +Heating therapy +some stretching exercises will be helpful for sure.
Health Tips
At home.. Frequent movements of neck up to full range even if its painful+hot water fomentation +pain killer gel +no cold exposure.