One side of the arm suddenly aching and weakness is considered to be due to frozen shoulder, cervical spondylosis, cerebrovascular disease and so on. 1. Frozen shoulder: Chronic strain caused by degeneration of soft tissues of shoulder joint or long-term over-activity triggers damage to the shoulder joint capsule and surrounding ligaments, tendons, bursa, etc., which is manifested as pain, and with the aggravation of the pain, it causes limitation of the shoulder joint activities, which can lead to sudden weakness of one side of the arm. 2. Cervical spondylosis: Degradation of cervical intervertebral discs or prolonged bowing of the head, protrusion of the nucleus pulposus or proliferative organisms, compression of the surrounding nerve roots, which can be manifested as numbness and weakness of one side of the arm, or even cause the falling of things. 3. Cerebrovascular disease: cerebral hemorrhage or cerebral thrombosis, obstruction of blood and oxygen supply can cause damage to the motor region of the brain, causing motor disorders on one side of the arm, which can be manifested as hemiplegia and weakness of one side of the limb. Sudden weakness of one arm may also have other reasons, it is recommended to go to the hospital in time, improve the examination to clarify the cause of the disease, and then give targeted treatment or therapy.