As detailed in How does the bounty system work?, when you place a bounty, after a week plus a grace period if no answers have +2 on votes or you haven't granted the bounty, the bounty simply expires and you've lost the points.
This is why the bounty page gives you notes that once you issue a bounty it isn't going to be able to be recovered/refunded.
This being said, using bounties to entice people to give detailed answers is not always the best solution - bounties tend to also incite lower-quality answers/posts as well.
