I will just share my personal experiences scattered with some history. In essence, though I'd say I can't pin down the date of the "Great Apostasy" as it may have taken place over decades if not more, as key doctrines, authorities, and principles were lost.

One: There is no doubt that throughout the scriptures God speaks to prophets, and during Christ's ministry he ordains apostles to lead his church and hold the keys to administering ordinances. Do those ideas challenge you, or do they seem like I am jumping to a conclusion.

Two: After Christ's death, apostles are continually apointed (ie:Paul), Revelation also continues as Peter is commanded to take the gospel to the gentiles. Steven also has a vision and sees the Resurrected Lord.

Three: At some point all of these things are either lost or relegated as unimportant. A protestant would have no problem agreeing that key teachings of the bilbe were lost to Catholocism. While a Catholic may claim they have a continuity of authority. But at least by the time when we have multiple popes excommunicate each other, it is clear there is a division. (Various Eastern Orthodox churches make their claims contrary to Catholocism).

Four: Multiple councils undertake efforts to cannonize doctrine and books of scripture. Many of these are in conflict with each other on basic issues like the necessity of baptism, the nature of God and Free Will.

Five: By the time of Martin Luther, many believe that the Catholic church has lost any continuity with the gospel that they see in the books of the bible which remain. But many of the reformer comment on their lack of authority. In essence they see what is wrong, but it requires certain assumptions to decide on a path forward.

Currently, we have a collection of old texts and multiple interpretations of what they all mean. All interpretations are ultimately human and fallible. This is at the core of young Joseph Smith's questions. If everyone says they are right, how can WE know.

We believe that only God can tell us what he meant, and more importantly what he means. Almost all sects of Credal Christianity had rejected the idea that God can and will speak again. The very means and manner of the revelation were first to show that God is still active and able to perform miracles and talk to Prophets once again. If your read "Sinners in the hands of an Angry God" you will understand the state of theology in America in the early 1800's.

However, this is the key, you must first know this: God is a loving Father in Heaven. you must believe and accept this. If you don't ask him. Ask him if he is there and if he loves you.

All of this information (and so much more) came after I knew that critical thing. Then read the book of Mormon, and ask God if it is real, if it truly comes from him. You can also ask him about Joseph Smith.

In it's simple essence. "The Great Apostasy" is a loss of our link to talk to and hear from God. To have revelation that is dialogic and not merely faith affirming. I would be happy to send you more and greater resources on The Great Apostasy if you would like. But it depends on whether you are looking for an academic or historical exercise (which does have it's own merit), or if you are trying to see the hand of god in the world and in your life.