Unfortunately sacrificed heroes cannot be recovered. As Empires & Puzzles is an online multiplayer game, reversing in-game content would provide you an advantage, which wouldn't be fair to other players. Same rules apply to every player.
Although you cannot recover your hero, keep in mind you do get an advantage by sacrificing heroes.
Please make sure you lock all your valuable heroes by using the favourite hero button. The game game is designed to ensure accidental hero sacrifices don't happen.
You can save a favourite hero by:
1. Selecting Heroes
2. Selecting the Hero you want to mark as favourite and
3. Tapping the Favourite button on the top left hand corner of the screen.
By following these steps, you can be sure not lose any favourite heroes in the future.