Hello, Thank you for your question. I would like to have more information regarding any history of trauma or previous fall, or medication that you use or even your age and profession. Anyway, based on your signs I would first focus on the shoulder problems such as rotator cuff tear that may give the signs that you explain. Rotator cuff help the arm to elevate and can tear after a fall or even gradually from degeneration in older ages. The onset of it is not so much immediate as you describe. An MRI of shoulder would confirm or exclude this diagnosis. The second reason might be a nerve problem of brachial plexus that originates from the neck. Nerve compression syndrome of the cervical spine from herniated discs may cause such picture as you describe in some cases. Third cause cause might be from the disease of the brain such as tumors , stokes , bleeding , aneurisms etc. Again an MRI of Head and Neck would clarify your situation and exclude most of serious conditions affecting you. I wish you quick recovery.