This question is a very close to question How could a mummy come back to life?, but I hope that not duplicate.
Undead = alive, because dead = after death, un = negation. But they are alive in a bit different way.
There young ghost girl (she is about 11 years old, but for more than 400 years of ghost life) accepts flower (exactly daisy) to get real life back.
I used a different way, to take very magic armor (made of scales of white dragon, one of the most magical being in the Old world) on.
And I think what else way could be used to bring undead back to real life. And of course, I accept that some undead (like skeletons or zombies) probably cannot be brought back to real life.