In which episode does Jessica come back alive?

Jessica never really 'came back to life'.
In the episode "What Is And What Should Never Be" 2x20, Jessica is alive because in Dean's wish-world she never died. But that all happened in his head, it wasn't real.

In the episode "Free to Be You and Me" Sam dreams of Jessica but it's actually Lucifer, telling Sam that he's his true vessel.

It was in the second season the 20 episode "What is and what should never be".
Did you mean this?

If you mean in season 5, it will be in episode 3:) the episode's called: Free To Be You And Me.
Kripke told in an interview, here's the link:
link