Once Upon a Time is a rare Instant card from Mystery Booster 2 (MB2), collector number 71. Current market price: $0.55, TCGplayer market prices via Scryfall.
If this spell is the first spell you've cast this game, you may cast it without paying its mana cost. Look at the top five cards of your library. You may reveal a creature or land card from among them and put it into your hand. Put the rest on the bottom of your library in a random order.