Searching a library for something, failing to find it, the later discovering it was there
You are never required to actually find a specific card when you search your deck, so if you don’t find it when you look, you have simply missed your chance.


  701.18b If a player is searching a hidden zone for cards with a stated quality, such as a card with a certain card type or color, that player isn’t required to find some or all of those cards even if they’re present in that zone.


So there is no difference between simply choosing not to find any Forest, or being unable to. 

Note that this works differently if a card has you search for cards in general, such as Demonic Tutor. In that case you actually must choose a card.