In spite of Jesus Christ’s great influence and many miracles, He was ultimately rejected and crucified. After his death, His brave and faithful apostles carried on without Him, baptizing new members, and starting various congregations.
Regardless of the valiant efforts of Christ’s apostles and their faithful followers, the original church that Christ restored began to fade away. Members faced severe persecution and all but one of the apostles were martyred. This is a period called the Great Apostasy, when there was a “falling away” ( 2 Thessalonians 2:1-3 ) from the gospel Christ organized. The apostolic authority to bestow priesthood keys and to receive revelation for the Church was lost along with many precious teachings. Errors about His teachings crept into the church resulting in conflicting opinions and lost truths. This period is what we call the Great Apostasy.
Talk Source: http://mormon.org/restoration