You never get a refund for a bounty. If there is still no answer at the end of the bounty period, the reputation is lost.
If there are answers but you don't award the bounty, half the amount will be awarded automatically to the eligible answer with the highest score (the oldest in case of a tie), if there is one. An answer is eligible if it was posted during the bounty period and its score when the bounty ends is at least +2. As an exception, if you accept an answer during the bounty period, this answer will be awarded the full bounty (unless you manually award the bounty to a different answer). If there is no eligible answer, no reputation is awarded automatically.
You have 24 hours after the end of the 7-day bounty to award the bounty (you can award it earlier if you wish). The automatic award or definitive loss kicks in after that grace period.
For more information, see the bounty FAQ on the main Stack Exchange meta site.