We as human beings are such wonderful creatures. We are given abilities that no other being has. Since I also am a human being, I know what it is like to lose someone you dearly love. Is it possible to restore lost happiness after losing them? It depends on what you mean by "lost happiness". If you mean the same happiness as when that person was with you, I would say no. You cannot re-live exact moments of the past. One thing that is constant in life is that life is dynamic, and ever changing. Even if that person was still with you today, the happiness you would enjoy today would not be the same as it was several years ago.
If, on the other hand, you mean, "Can I become happy after the loss of a loved one?", then my answer is a definite "YES". There are many reasons to find happiness after the loss of a loved one. You do not have to wallow in misery for the rest of your life. However, becoming happy is a choice. You choose what you will focus on, and what you will think about.
One reason to become happy is to look at the motive why you are sad in the first place. Is it because you think that your loved one is suffering? That they cannot enjoy the pleasures of this world anymore?
Most of the time, we are sad because we miss the love and attention that person used to give to us. If you truly look at these motivations closely, you will see that at the root, they are selfish and self-centered.
Rejoice at the moments of joy you spent with that person. Count them as blessings. Celebrate their life with you. Know that whereever they are at, they are in a better place than here, and be glad for them.
If that person gave you such joy at being around them, then do what they did to you to others. Make the lives of others a joy. That way, you honor the memory of the dearly departed one and enrich the lives of others with the experiences you had. If you truly love the loving one, as you say, then allow his/her life to continue to resonate through you. In this way, that person's influence will continue on this earth, in spite of the fact they have long since departed.
Perhaps some of my articles might be of assistance if you are still struggling with this issue.
I wish you the very best.