Slipped disc occurs when an intervertebral disc impinges on a ligament or narrows the space through which a nerve leaves and enters the spine; overuse or degenerative change precedes most disc problems, the awkward bend or twist being the final straw. In most cases symptoms are sudden, severe pain, usually in middle or lower back, and inability to straighten up or bend the neck forwards; there may be severe pain down one or both legs. Sometimes, however, symptoms onset gradually.
Let the person lie down in whatever position feels most comfortable; lying on the side, with the knees tucked up and a cushion between them, is sometimes more comfortable than lying flat on the back with the knees flexed. See your doctor within 12 hours. Rest is usually the best medicine; the back should not be manipulated until pain has abated.