As detailed in https://meta.stackexchange.com/questions/16065/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 intice people to give detailed answers is not always the best solution - bounties tend to also incite lower-quality answers/posts as well.