How can I deal with an unresolved relationship with a person who is no more or I can't get in touch with?

In such a case, you can write a letter addressing the other person truly expressing your deepest feelings. Once you are done writing you can keep the letter with you, or even tear it and throw it away. That is your choice.

Remember it is not important that the other person receives your letter or responds favourably. The shift happens when we write. that allows us to resolve the strain from our side. And thus, even if the other person is no more or you dont have their contact details, the process is still as powerful.