The following is a simplified generalization, but I think it is a useful and reasonably accurate one:
Martin Luther and other Protestant leaders of the Reformation realized very clearly that they did not have "The Priesthood" as it was understood at that time within Catholicism. In rejecting Catholicism, they also rejected the idea that God's authority was vested exclusively and totally in a few people through an ordination process (who were the only people required to read and know the word of God, and, therefore, stood as intermediaries between the people and God) - replacing that concept with the idea that God's authority was vested purely in His word (The Bible) and all true believers had the ability to read His word and act according to their own understanding of it ("the priesthood of believers").
Of course, this has been limited over time to be only those understandings of the Bible that don't contradict their own interpretations - which is ironic, given the foundation of the Reformation. Thus, in their construction, all who are "true believers" (not Mormons, Jehovah's Witnesses, Catholics, and other deluded cultists) have the right to act in God's name (as true Christians), but God's actual authority lies only in the Bible.
So, the overall Mormon view of authority is kind of a combination of Catholic and Protestant views - since we maintain a formal, capital "P" Priesthood for the performance of binding ordinances but couple it with the general idea of a lower case "p" priesthood of believers who can read and act according to God's universal word, even though we don't talk about it in those terms. The "additional" aspect within Mormonism is that it posits that "His universal word" includes more than just the Bible - that it includes whatever he has said to all (truly universally) - and that individual believers actually can receive His unique word to them, even if it contradicts, occasionally, His universal word to all.
Therefore, in a nutshell, the Great Apostasy, as it is defined within Mormonism, began when the apostles no longer were able to gather and replace those who were being killed (to continue the ordinance-performing Priesthood) and was further entrenched when the priesthood of believers was eliminated by the Catholic Priesthood organization that removed from them the right and ability to read God's word and interact with God directly within their own spheres. The Reformation addressed the foundation of the second of these issues (allowing regular believers to interact directly with God through exposure to His word), while the Restoration addressed the extension of the second issue (re-establishing truly personal revelation as a universal right) and the first issue (re-establishing binding Priesthood ordinances).
4 hours ago