The job of your spinal cord is to deliver messages from the brain to the rest of the body. It’s filled with thousands of nerves, transmitting rapid signals every second of the day. These nerves are very delicate and, unlike most other cells, are irreparable if they become damaged or torn. Luckily, they are protected by the vertebrae in your spine. It usually takes a lot of impact or trauma to completely break through these bones and make it to your nerves. At most, an impact usually causes bruising, swelling and/or small fractures.
These injuries can cause temporary paralysis and other issues, but with proper treatment and rehabilitation, you could be back on your feet in a few weeks to a few months. But in more severe cases, your injuries could become permanent. The likelihood of a nerve being severed or otherwise directly damaged is low, thanks to the aforementioned vertebrae. But, if swelling or other side effects of trauma go untreated, it could cause a loss of blood flow to the nerves. That causes nerve death, which (at least for now) is permanent.
New research is constantly being conducted to remedy these kinds of injuries. Stem cells have been thought to be a solution for spinal cord injuries. However, due to funding and moral reasons, research has been slow. For now, “complete” spinal cord injuries, or ones in which the nerves are completely damaged, are permanent.