There has been much discussion about this sort of situation, and the question is whether there is a repairable game state, and that's what we encourage judges to look at.
In the situation you describe, generally the character comes back, as that is immediately repairable. Do you get to draw? No, that's optional, you missed it.
But what if it were a mandatory draw? Or what if the opponent wanted to be able to target that hero with an ability already played in that battle? This is where we get into what is repairable and what is not. If you realize in the Prep Phase that you didn't trigger Areopagus, then everyone should draw one, you can repair that. However, your Abom in a territory doesn't go off, because you missed the trigger for it. Now it's Discard Phase and you realize Areopagus didn't go off...that's less repairable, because that would have influenced the battle and everything your opponent did. Probably should rule that it doesn't happen that turn. If your opponent played Plot in the battle and would have hit your Provisioned hero had they been back, is that repairable? That's a tougher question. It really depends on the game state and what a judge sees as they look at it.
Really, it's a judge's call, and it should be based on whether you can repair the game state without the correction being a major change to the game. At Nats in T2-MP this past year, I had Abom put in my territory, and that person also laid down some Greeks. I only noticed Demetrius because he is my bane (stupid Seized...), but they also had a Stoic I just didn't notice. I Passover Hymn'd the Abom away, and we were two turns further into the round before the Greek player realized I shouldn't have been able to. But my turn was past, during which I used 3-4 draw abilities since Abom wasn't a factor, and there was really no good way to go back and repair that game state. Had Abom still been there, I should have had a lot blown up, but had it been there I wouldn't have used the draw. The judge came by, and ruled it should stay discarded, it wasn't repairable.
If you're playing a friendly game, try to reach a consensus on what should happen based on the game state as above and what you can repair. If you really can't agree and there are two options that make a decent amount of sense, roll-off.