Dropped shoulder occurs due to paralysis of
Elevation of the scapula (as in shrugging the shoulders)The movement is brought about by the upper fibres of the trapezius and by the levator scapulae. It is associated with the elevation of the lateral end, and depression of the medial end of the clavicle. Spinal accessory nerve supplies trapezius muscle and may get injured in trauma or damage during surgery, resulting in weakness of the trapezius muscle and drooping of shoulder.
Deltoid
Trapezius
Teres minor
Serratus anterior
Upper limb bones and muscles (proximal region) & Scapular movements