What does Isaiah 8:20 mean?
Judah has been looking for answers from the spirits of the dead. They seek these beings—against God's explicit command (Leviticus 19:31)— through the mediums and necromancers. This is a practice Isaiah has unequivocally rejected. God reveals some things to His people, and there are some things He does not. In Scripture, divination, necromancy, witchcraft, and mediums were all attempts to gain information God had not otherwise made available. That means wasting time, at best—or contacting spirits antagonistic to both God and men, at worst.The Lord has asked why people would not bring such questions to their God (Isaiah 8:19). In this instance, answers are fully available to all who are truly willing to hear. Those truths could be seen and understood by the living. Isaiah is stating how they do not need to inquire of the dead. He seemingly shouts that God has revealed Himself through the prophets and through His Word.
The problem is not that God is hiding the truth, it is that His people will not receive it. In other words, Isaiah says that those who will not hear the revealed word of God live in darkness. They reject the light and the light does not come. The day of their understanding of the truth never dawns.
Paul describes something similar in Romans 1, pointing to creation itself as revelation of the truth of God. Despite such truth being on full display before their eyes, the unrighteous refuse to hear it. They attempt to explain away the creation in other ways (Romans 1:18–22).