Can damaged fingernails ever be repaired back to normal?

Yes: Once you find out the cause and treat accordingly, fingernails should improve. There are local causes, ie- fungal infection and general causes in the form of anemia or vitamin/electrolyte deficits. Not to mention pure skin conditions that might affect primarily your nails.

How were they damage: Depends on the nature of the damage. Nails are not a living tissue. They cannot repair themselves. If the tissue under the cuticle, where new nail is formed, is intact, a new nail will grow out. It takes 6 months to grow a complete fingernail, 12 months for a toenail.