The Bhagavad Gita in the sixth chapter of the Gita Arjuna asks this precise question to Krishna what happens to a person who is not enlightened. "Is it all lost do I have to begin from the world be imagine starting with Mandu Kyo Penishad all over again" He says such persons after death they will go to the higher heavens, he says thousands of years there ushitwashashwati some are many years long years.
Swami Sarvapriyananda teaches one of the shortest and most profound upanishads accompanied by Gaudapada's Karika. This lecture covers Karika 22-23 and Mantra 12.