Teshuva Without Appeasement

What is someone to do if they can not find the person they commited a sin to, (to ask forgiveness) and there is no hope of finding them to say sorry? Will it ever be possible to do complete teshuva (repentence)? Thank You.

Indeed a difficult situation. The Code of Jewish law writes (Orach Chaim 426:2) that if the person that you offended has died one must bring 10 people to the graveside and ask forgiveness. If you caused damage to a group or to the public and don't know who to compensate you should engage in works of public good. In a case where you have no idea who the person is that you hurt, you should
1. make efforts to find the person and ask for forgiveness - e.g. put up a sign, or ad in newspaper to ask the person to contact you.
2. Try to compensate the group from which this person came for any damage that you caused
3. Pray to God that the person should forgive you.
If you put in your best effort and still do not succeed, do not give up hope. God is merciful and wants repentance not vengeance. It is within He power to inspire your victim to forgive those who hurt him and then your repentance will be fully accepted. Remember also that the Talmud says that one is not blamed or punished for circumstances beyond their control