In Elder Sign, if you need to draw from a deck and there are no cards left, do you lose the opportunity?
If there are no cards left, then you do not gain anything.  Ideally, you should be making use of these cards as often as possible to ensure that future gains do not get wasted due to not having cards available.  

(Remember that all discarded cards go immediately to the bottom of their appropriate decks -- so you may want to spend spells freely in encounters that have spells at stake -- they'd then be at the bottom of the deck, and available as part of your reward.)

Exact quote from page 13 of the rules:


  After all dice on a Spell card are removed, the player returns the
  card facedown to the bottom of its deck.