Are there any examples of a fictional Western mortal hero that became a god, or came back from death?
What an interesting question! I assume you're referring to Western literature, rather than, say, movie characters or comic book characters. So, I've been pondering this for the last hour or so, and only one name comes to mind--and even he may be cheating the definition of "mortal."
The simplest definition of the word is "subject to death." If that's the standard of mortality, my character suggestion fits. He died, then he came back to life. Are you ready?
Gandalf the Grey, later known as Gandalf the White, from The Lord of the Ringstrilogy. He is the wizard who spends his life in a battle (both active and passive) against Sauron. He mentors Frodo and tries to gather the forces of good against the forces of evil. He falls to his death in Moria while battling a Balrog. Gandalf the White returns to life on Middle Earth, for just a short time, in order to assist in the final defeat of Sauron. Once the battle is won, Gandalf simply walks away.
The debate, of course is whether or not a wizard is a mortal. Guess I'd just say this: a wizard is not a god, and he is, to some degree apparently, subject to the laws of life and death. That makes him a mortal...I think.
That's it. Wish I had more for you.