If the bounty manager is no longer communicating, there's not much you can do. If you visit his or her profile and see that he or she has a low rank on the forum and is relatively new as a bounty manager, you can give him or her a red trust if you have evidence of non-payment for your bounty. However, make sure that your action is based on the forum's policy and has sufficient grounds, as giving a red trust has a profound impact on a person's reputation. On the other hand, you may choose to wait for a few days or weeks, as there might be a personal reason for his or her offline status