Frankl was a prisoner in a Nazi concentration camp and wikipedia tells us:
It was due to his and others’ suffering in these camps that he came to his hallmark conclusion that even in the most absurd, painful and dehumanized situation, life has potential meaning and that therefore even suffering is meaningful. This conclusion served as a strong basis for Frankl’s logotherapy. An example of Frankl’s idea of finding meaning in the midst of extreme suffering is found in his account of an experience he had while working in the harsh conditions of the Auschwitz concentration camp:
… We stumbled on in the darkness, over big stones and through large puddles, along the one road leading from the camp. The accompanying guards kept shouting at us and driving us with the butts of their rifles. Anyone with very sore feet supported himself on his neighbor’s arm. Hardly a word was spoken; the icy wind did not encourage talk. Hiding his mouth behind his upturned collar, the man marching next to me whispered suddenly: “If our wives could see us now! I do hope they are better off in their camps and don’t know what is happening to us.” That brought thoughts of my own wife to mind. And as we stumbled on for miles, slipping on icy spots, supporting each other time and again, dragging one another up and onward, nothing was said, but we both knew: each of us was thinking of his wife. Occasionally I looked at the sky, where the stars were fading and the pink light of the morning was beginning to spread behind a dark bank of clouds. But my mind clung to my wife’s image, imagining it with an uncanny acuteness. I heard her answering me, saw her smile, her frank and encouraging look. Real or not, her look was then more luminous than the sun which was beginning to rise.
A thought transfixed me: for the first time in my life I saw the truth as it is set into song by so many poets, proclaimed as the final wisdom by so many thinkers. The truth — that love is the ultimate and the highest goal to which man can aspire. Then I grasped the meaning of the greatest secret that human poetry and human thought and belief have to impart: The salvation of man is through love and in love. I understood how a man who has nothing left in this world still may know bliss, be it only for a brief moment, in the contemplation of his beloved. In a position of utter desolation, when man cannot express himself in positive action, when his only achievement may consist in enduring his sufferings in the right way—an honorable way—in such a position man can, through loving contemplation of the image he carries of his beloved, achieve fulfillment. For the first time in my life I was able to understand the meaning of the words, “The angels are lost in perpetual contemplation of an infinite glory….” 
Another important conclusion for Frankl was:
If a prisoner felt that he could no longer endure the realities of camp life, he found a way out in his mental life– an invaluable opportunity to dwell in the spiritual domain, the one that the SS were unable to destroy. Spiritual life strengthened the prisoner, helped him adapt, and thereby improved his chances of survival.
Frankl’s concentration camp experiences thus shaped both his therapeutic approach and philosophical outlook, as reflected in his seminal publications. He often said that even within the narrow boundaries of the concentration camps he found only two races of men to exist: decent and unprincipled ones. These were to be found in all classes, ethnicities, and groups. Following this line of thinking, he once recommended that the Statue of Liberty on the East coast of the US be complemented by a Statue of Responsibility on the West coast, and there are reportedly plans to construct such a statue.
Frankl’s approach is often considered to be amongst the broad category that comprises existentialists. Frankl, “who has devoted his career to a study of an existential approach to therapy, has apparently concluded that the lack of meaning is the paramount existential stress. To him, existential neurosis is synonymous with a crisis of meaninglessness”.
He is thought to have coined the term Sunday Neurosis referring to a form of depression resulting from an awareness in some people of the emptiness of their lives once the working week is over. This arises from an existential vacuum, which Frankl distinguished from existential neurosis.