Was there ever a time when records of the Lord were hidden and later found? Yes. Consider the account of Hilkiah (relevant points footnoted):
14 And when they brought out the money that was brought into the house of the LORD, Hilkiah the priest found1 a book of the law of the LORD given by Moses.
15 And Hilkiah answered and said to Shaphan the scribe, I have found the book of the law in the house of the LORD. And Hilkiah delivered the book to Shaphan.
16 And Shaphan carried the book to the king, and brought the king word back again, saying, All that was committed to thy servants, they do it.
17 And they have gathered together the money that was found in the house of the LORD, and have delivered it into the hand of the overseers, and to the hand of the workmen.
18 Then Shaphan the scribe told the king, saying, Hilkiah the priest hath given me a book. And Shaphan read it before the king.
19 And it came to pass, when the king had heard the words of the law, that he rent his clothes.2
20 And the king commanded Hilkiah, and Ahikam the son of Shaphan, and Abdon the son of Micah, and Shaphan the scribe, and Asaiah a servant of the king’s, saying,
21 Go, enquire of the LORD3 for me, and for them that are left in Israel and in Judah, concerning the words of the book that is found: for great is the wrath of the LORD that is poured out upon us,4 because our fathers have NOT KEPT THE WORD of the LORD, TO DO AFTER ALL THAT IS WRITTEN in this book.5 (2 Chronicles 34)
1) A holy book, the “Law of the Lord” was hidden and later discovered.
2) After comparing the written word with what they were doing, the king rent his clothes in disgust.
3) It is a persons’ right to inquire of God regarding any book that claims to be from Him.
4) The realization was, they had not faithfully kept what the scriptures said. Without the written word there is nothing to prevent things from becoming lost due to the nature of memory loss, or falsehoods injected.
5) This was not the first, nor would it be the last time that the Word of the Lord was not faithfully kept. The key word here is “all”; to receive God’s favor, a person or people must faithfully keep God’s established law which cannot accurately be accomplished when the law is missing.
Scriptures set a precedent for the discovery of hidden sacred records and what to do when they are found: ask God, and if true, then OBEY what they say. We see the same admonition in The Holy Book of Mormon:
2 And I seal up these records, after I have spoken a few words by way of exhortation unto you.
3 Behold, I would exhort you that when ye shall read these things, if it be wisdom in God that ye should read them, that ye would remember how merciful the Lord hath been unto the children of men, from the creation of Adam even down until the time that ye shall receive these things, and ponder it in your hearts.
4 And when ye shall receive these things, I would exhort you that ye would ask God, the Eternal Father, in the name of Christ, if these things are not true; and if ye shall ask with a sincere heart, with real intent, having faith in Christ, he will manifest the truth of it unto you, by the power of the Holy Ghost.
5 And by the power of the Holy Ghost ye may know the truth of all things. (Moroni 10)
Many people have tested that promise, and in every case where its requirements were met (i.e., asking with a sincere heart), the individual received an answer from God. For this reason, The Holy Book of Mormon is a powerful introductory tool for how to communicate with God.
The second part where people fail, like the Israelites, is by not doing “all that is written” in The Holy Book of Mormon (which does not contradict The Holy Bible). Many are introduced to The Holy Book of Mormon by the Mormon Church (which is not faithfully following its precepts) but are then taught non-Biblical teachings its founder introduced, long after he did his part to bring it forth.
Many Jews currently sit in darkness because of the same thing; they are following rabbinical teachings, not scriptures. Mormons follow their Doctrine & Covenants and temple teachings, and not the Holy Bible and Holy Book of Mormon. They use the power of The Holy Book of Mormon for gaining converts and infer its confirming power includes all that Joseph Smith did afterward. The Mormon Church and its leaders continue this today (i.e., they are all good, all God-approved, all infallible).
The Mormon Church is not based on The Holy Book of Mormon:
“Despite the effort that went into the translation, Joseph Smith did not make the book the foundation of the church” (LDS Professor Richard L. Bushman, Joseph Smith and the Beginnings of Mormonism, 1984, p. 142).
Mormon scriptures say they are under a curse for not doing what The Holy Book of Mormon says:
“I have noted within the Church a difference in discernment, insight, conviction, and spirit between those who know and love the Book of Mormon and those who do not. That book is a great sifter (“Jesus Christ-Gifts and Expectations,” Christmas Devotional, Salt Lake City, Utah, 7 December 1986; Teachings of Ezra Taft Benson, pp. 60-61).
“Among the hallmarks of Ezra Taft Benson ‘s tenure as president of The Church of Jesus Christ of Latter-day Saints was his stirring summons to reemphasize the Book of Mormon.
‘My beloved brethren and sisters,’ he said during the October 1984 general conference, ‘for some years now I have been deeply concerned that we are not using the Book of Mormon as God intends. As I participated in the Mexico City Temple dedication, I received the distinct impression that God is not pleased with our neglect of the Book of Mormon. In the 84th section of the Doctrine and Covenants, the Lord decreed that the whole church was under condemnation, even all the children of Zion, because of the way they treated the Book of Mormon. ‘And they shall remain under this condemnation until they repent,’ said the Lord, ‘and remember the new covenant, even the Book of Mormon’ (Doctrine and Covenants 84:57)'” (Dan Peterson, “Embracing the power of the Book of Mormon,” Deseret News, January 5, 2012).
54 And your minds in times past have been darkened because of unbelief, and because you have treated lightly the things you have received-55 Which vanity and unbelief have brought the whole church under condemnation.56 And this condemnation resteth upon the children of Zion, even all.57 And they shall remain under this condemnation until they repent and remember the new covenant, even the Book of Mormon and the former commandments which I have given them, not only to say, but to do according to that which I have written- (D&C 84:54-57, 1832)
If you want to convert a Mormon to Christ, use the Book of Mormon and Bible. If you want to convert a Jew to Christ, use the Torah and other books from the “Old Testament” such as Isaiah (see the ministry of Messianic Rabbi Zev Porat for many examples).