The period of time when the true Church no longer existed on earth is called the Great Apostasy. (Gospel Principles, Lesson 16: The Church of Jesus Christ in Former Times) Priesthood authority was restored to the earth by Jesus Christ through the prophet Joseph Smith. That has a lot of big words. What does it mean?
Pretend that this building (Show the simple block building without the "Authority/Priesthood" block) represents the Church as Jesus organized it when he was on the earth.
The church cannot be organized correctly without the priesthood (add the "Authority/Priesthood" block).
The church must be “built upon the foundation of the apostles and prophets, Jesus Christ himself being the chief corner stone" (Ephesians 2:20). If the priesthood is gone, the church is not built the way Heavenly Father wants it to be organized.
After Jesus returned to Heaven, wicked people tried to destroy the church. Many people changed the way the church was organized. (Move the blocks around)
People taught their own ideas instead of Heavenly Father's. Jesus's Apostles were killed. The perfect church Jesus organized was broken. (Knock the block tower over)
The priesthood was finally taken from the earth because the people were so wicked. (Find "Authority/Priesthood" block and take it back.)
Many people tried to rebuild Jesus's church, but they couldn't build it just right. Without the priesthood, the church can't be built correctly. (Build a building without the "Authority Priesthood" block.)
People lived a long time without the priesthood. They forgot how much Heavenly Father loves us. They forgot and changed His commandments. They changed Heavenly Father's ordinances, like baptism and the sacrament. They even forgot why we came to earth. People were very confused.
Heavenly Father knew the Great Apostasy would happen, and He prepared a way for the gospel to be restored -- He prepared a way to rebuild His church correctly.
When he was 14 years old, Joseph Smith asked Heavenly Father which church he should join. The Savior told him not to join any church because the true Church was not on the earth. Jesus told Joseph that that he would help organize the true church again. John the Baptist came in 1829 to ordain Joseph Smith and Oliver Cowdery to the Aaronic Priesthood (see D&C 13; D&C 27:8). Later, Jesus' Apostles Peter, James, and John, gave Joseph and Oliver the Melchizedek Priesthood (see D&C 27:12–13). Through this Restoration the priesthood was returned to the earth! (Build the tower correctly, with the "Authority/Priesthood" block)
The church was organized again like Heavenly Father wants it. Heavenly Father has promised us that since the church has been restored, it will never be destroyed again (Daniel 2:44).
(see Gospel Principles, Lesson 16: The Church of Jesus Christ in Former Times and Gospel Principles, Lesson 17: The Church of Jesus Christ Today)
** This would be an especially effective object lesson if when you removed the "Priesthood Authority" block the structure collapsed on its own. -- Jenny **
** You may also want to consider labelling some of the other blocks "apostles" or "seventies" or whaveter if you are using this lesson to teach the offices of the priesthood. -- Jenny**