In dealing with the question of what happens to those who never got a chance to hear about the Messiah in the Old Testament times, or Jesus Christ today, it helps to first establish who we are talking about.
We are presumably referring to people like the Native Americans, Aboriginal Australians, Polynesians, and other groups separated from the Old World continents of Europe, Asia, and Africa, by a significant barrier of ocean water, who presumably had no regular contact with the Old World during many hundreds, or thousands, of years.
Today, we would presumably be talking about the average North Korean, and other such people which are significantly limited in their freedom to study religion, use the internet, or access a Bible.
In order to understand these peoples' situation, we need to reach a conclusion on how a person gets to Heaven. If all relatively good people go to Heaven, then it wouldn't matter how isolated any people group has been throughout time.
However, God states very clearly that in order to go to Heaven, you must believe in Jesus Christ (John 3:18, 3:36, 8:24, 14:6, Mark 16:16, Acts 4:12: 1 John 5:12). The Bible also teaches that Salvation by faith has always been the only way to be saved, even before Christ's incarnation, as covered in this article.
Finally, the Bible teaches that all children, and the severely mentally ill, are saved, as is explained in-depth in this article.
Putting these three things together tells us that if any person throughout history lived to the age of accountability, they must have believed the saving proposition, regardless of where or when they were born. If they did not, they died in their sins, and will go to Hell.
However, the Bible is also clear in teaching that anyone who genuinely seeks for Eternal Life from the Lord will find it (Matthew 7:7-8, Luke 11:9-10, Hebrews 11:6, Deuteronomy 4:29, Psalm 9:10, Proverbs 8:17, Acts 17:26-27: 1 Chronicles 28:9, Romans 2:5-8, John 7:16-17).
How is that possible, though? If they are uncontacted by Old Testament Jews, or Christians, how do they receive an answer when they seek for the truth? And how do they know to seek for the truth in the first place?
To begin, we know that once someone reaches the age of moral accountability, their God-given conscience kicks in. This is the natural law of God, written in the hearts of all people, regardless of whether they have special revelation:
Romans 2:14-15
14 For when the Gentiles, which have not the law, do by nature the things contained in the law, these, having not the law, are a law unto themselves:
15 Which shew the work of the law written in their hearts, their conscience also bearing witness, and their thoughts the mean while accusing or else excusing one another;)
This provides the impetus for people to begin to search for answers as to how they can be justified before God. There is also the testimony of nature (Psalm 19:1), which draws people to want to know the God who made the universe. Both sources of natural revelation have lead people to seek to know God.
The way that God could get the Gospel message to such people, even if isolated, is primarily through dreams and visions. We see this frequently in the Old Testament (Genesis 20:3-7, 41:1-7, Numbers 12:6, Job 4:12-17, etc.).
He could have also made prophets out of local converts, once they believed. We can speculate on exactly what He did, but since there is no record of it, we can't be dogmatic, beyond saying that if any person from any point in history sought Him, they found Him. God has never left mankind utterly without a witness, and is interested in rewarding those who seek Him (Hebrews 11:6).
Finally, recognize that absence of evidence is not evidence of absence. It's ignorant to the point of foolishness to think that we have even 0.1% of the historical record. Most of history, especially for dispersed people groups, is completely lost to time, never mind for random, insignificant individuals from within those groups that may have received a vision before they died, which completely contradicted their people groups' Animism.
In summary, even those who have never heard about Christ could have heard about Him, had they followed the natural revelation that God has given to all mankind. The God of the universe is more than capable of getting the truth to anyone who wanted it, from any location, at any point in history.
God, despite requiring faith, is not unjust, and has never put anyone in a hopeless situation, where they are utterly unable to be saved, should they live past the age of accountability. All that seek find, and that has always been the case.