The Shaman hexed Nadia into a mysterious illness that quickly made her fall into a coma.
In Death Didn't Become Him, Bo asked The Lich for the meaning of the nail and he told her (with Lauren by her side), "That nail is an African shaman's cursing nail. Somewhere there is a piece of wood with Nadia's nail in it. Remove the nail and she will most likely wake up."
Nadia was freed from the curse and revived after Bo located the shaman and removed the cursing nail from the post he had hammered it into.