>I wonder what caused the neck down paralysis initially

Nerve damage.

>and how it recovered

The nerves healed.

nerve damage to the spinal cord? or peripheral nerves? why did the nerve damage there take longer to reverse than nerve damage in the brain?

also, what do you mean by damage exactly? the axons presumably weren't damaged. the neurons were apparently not "online" but presumably didn't go through apoptosis. do you mean metabolic dysfunction?

Nerves in the CNS hardly ever heal or regenerate, tetraplegia is usually damage to the cervical nerves which are CNS