How do i get my normal toenail back after a severe black toenail?

Avoid more trauma: Assuming you mean the typical "black nail" which develops from either direct or repetative low grade trauma(ie: toe banging up against tip of shoe in runners), the "black" needs to grow out. If more trauma or pressure can be avoided, the color will grow out as the nail grows. This may take many months.

See below: You need to trim the toenail back as far as possible. Soak the toe in hydrogen peroxide to loosen the debri on the nail bed. Start using a topical antifungal to the nail bed. It will take almost a year to grow back the nail.