After king Solomon, the kingdom of Israel was divided into two kingdoms, the kingdom of Ephraim to the north and the kingdom of Judah to the south. Between 740 and 720 BC, the ten northern tribes were sacked by the Assyrians in a series of conquests (2 Kgs 17). The inhabitants were taken north. Over the subsequent years, some of them were scattered all over the earth, but a group of them retained their identity and were able to preserve that identity for generations. A knowledge of them has been hidden from the world, much like the knowledge of the descendants of Lehi has been hidden from most of the world. They have been known as the lost ten tribes, but they are not lost unto the Father, for he knoweth whither he hath taken them (3 Nephi 17:4). From the Book of Mormon, we learn that these ten tribes retained their own identity, were taught directly by the Savior, had their own prophets and their own records (2 Nephi 29:12-14).
Bruce R. McConkie
"The Lost Tribes are not lost unto the Lord. In their northward journeyings they were led by prophets and inspired leaders. They had their Moses and their Lehi, were guided by the spirit of revelation, kept the law of Moses, and carried with them the statutes and judgments which the Lord had given them in age past. They were still a distinct people many hundreds of years later, for the resurrected Lord visited and ministered among them following his ministry on this continent among the Nephites. (3 Ne. 16:1-4; 17:4.) Obviously he taught them in the same way and gave them the same truths which he gave his followers in Jerusalem and on the American continent; and obviously they recorded his teachings, thus creating volumes of scripture comparable to the Bible and Book of Mormon. (2 Ne. 29:12-14.)
"In due course the Lost Tribes of Israel will return and come to the children of Ephraim to receive their blessings. This great gathering will take place under the direction of the President of The Church of Jesus Christ of Latter-day Saints, for he holds the keys of 'the gathering of Israel from the four parts of the earth, and the leading of the ten tribes from the land of the north.' (D. & C. 110:11.) Keys are the right of presidency the power to direct; and by this power the Lost Tribes will return, with 'their prophets' and their scriptures to 'be crowned with glory, even in Zion, by the hands of the servants of the Lord, even the children of Ephraim.' (D. & C. 133:26-35.)" (Mormon Doctrine, pp. 455-8)