This is a great question and can be answered in many different ways. Not all powers are the same and not all power decline in the same way. We live in a complex world and all we can say for certain is that powers rise and fall. They always have and always will. I will give an example of our current state of affairs in America.
So, if you look at the United States, it gained power through military prowess in World War II, technology, and finance. It also had great amounts of soft power, such as American culture, music, art, and the movies. For many decades, America was at the forefront in the world. It's power was unquestioned and it was the center of the world.
Its power was challenged when other nations began to question the idea of American exceptionalism. Some cultures began to see it as Americans exploiting situations for their own gain. Simultaneously, other power began to rise, such as China today.
I am not saying that America is on the verge of losing power, but it does not look great. If we continue to bleed in our economy and our politicians do not work together, we might implode internally.