How would a zombie come back from the dead?
Well first they'd have to be real
If its brain and spine is in tact
It technically can't unless magic is involved.
A virus controls the body from the brainstem and uses the body to transfer the virus to more hosts.
A virus can stack the nervous system, reanimating the body.
A zombie is supposedly resurrected by mystical means, such as witchcraft.
they were probably never dead in the first place