Hi Harper and welcome to the forum, Yes they can, if they are in the right environment, that means once whatever was causing them to damage is taken away, its a pretty slow process though but well worth the wait.
There is a lot you can to help this along, eg- B12, multi B, etc, i would follow Liza's advise, there is a great thread by David about Supplements and a mountain of other good info, good luck.
Brian