What could be the cause of neck pain and stiff neck muscles?

Neck pain: Cervical muscle spasm or strain. Do range of motion exercises, take ibuprofen, do yoga, get massage, don't stare at screens too long

Many possibilities: Pain reflects pain signals from nerve ending but does not indicate cause. The trigger can be mechanical, from sprains, trauma, nerve root irritation from vertebral erosion;or infectious as when viruses migrate into muscles during flu or during meningitis or tissue infections. The treatment would depend on identifying a cause.