"[H]e spake by the mouth of his holy prophets, which have been since the world began..." (Luke 1:70).
“Surely the Lord God will do nothing, but he revealeth his secret unto his servants the prophets” (Amos 3:7).
After the Savior's death and resurrection, many taught righteous principles based upon the Savior's teachings. However, due to the death of the apostles, the authority to act in God's name and to receive revelation on behalf of the people was lost from the earth.
Individuals could certainly receive answers to their own prayers and receive inspiration (revelation) in their own realm of stewardship (how to raise their families, what was the right choice for them as individuals, etc.), but there was no representative chosen by God to represent Him on the earth. Without a prophet on the earth, we could not receive instruction and guidance as it relates to the gospel in today's society.
In 1830, the fullness of the gospel of Jesus Christ was restored to the earth when The Church of Jesus Christ of Latter-day Saints was established. To find out more details about the restoration, please view the following video.
Today, we have a living prophet who leads, counsels, and guides us. His name is Thomas S. Monson.
I know that God has restored his gospel to the earth in it's entirety, including selecting a prophet to lead people back to Him.