I had a bounty on a question that had no answers, only comments. I put a bounty on it so people would answer it, and the bounty ended with no answers. I'm currently on the grace period and I don't know what to do. What do I do with my bounty?
The accepted answer to this question seems to suggest that in the situation where you cannot award a bounty (because there are no answers), the bounty effectively is lost - i.e. you don't get the unawarded rep back.
Just to clarify what Bounties do, Bounties do not guarantee correct or high-quality answers or any answers at all. Bounties simply highlight your question and therefore can lead to more exposure and better answers.
The first place to look for should be the bounty help page for you.
It very clearly mentions that:
All bounties are paid for up front and non-refundable under any circumstances.
That should make it clear that you cannot take back the bounty because it was paid right when you started the bounty and will not be refundable no matter what are the circumstances.
What if my bounty does not result in answers?
The help page also makes it clear that Bounty does not guarantee good answers, it only provides you a place in featured tab:
Part of what you’re “paying for” with the bounty is to get additional attention for your question, over and beyond what a normal question gets. A bounty does not guarantee a response, however, and reputation refunds are not available if no answers are received as a result of the bounty.
You paid for 7 days of a slot in featured tab, you got the slot and exposure.
As already suggested, if there are no answers, the bounty is lost. It was payment to get the premium slot, not to get answers.