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."
Lachlan told Bo that a Cursing nail can only be removed by a person that has complete and utter selfless love, which is exemplified by performing a life-changing favor for someone without them ever being aware of it. Bo located the Shaman, removed the Cursing nail from the post he had hammered it into, and freed Nadia from the curse (Masks).