A broken neck, or fracture of the cervical spine, is usually very serious but the seriousness depends on which bone(s) are broken and how they broke. The dangerous part of injuring your neck is the damage that is done to the nerves that run down the spinal column.
If an injury avoids damaging the nerves, this can be “fixed,” but if the nerves are damaged or severed, this may not be fixable and could, in fact, lead to death.