If the bounty manager is no longer communicating, there's not much you can do. I visited his/her profile and noticed he/she have a low rank on the forum and are relatively new as a bounty manager. You can give him/her a red trust if you have evidence of non-payment for your bounty. However, ensure that your action is based on the forum's policy and has sufficient grounds. 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/her offline status.