The same thing that happens when other animals die. Our remains would be no more special than the remains of anything alive today. We would decompose and be reabsorbed by the earth. Parts of us eaten by living bacteria, insects or animals so are then eaten by other living things. Parts of us may remain undisturbed, buried under layers of debris and sediment until the next geological upheaval brings layers of rocks in which our remains are to emerge to the surface again as fossils, parts of rock, mineral, oil, lava spurting out of a volcano or just plain dirt. The story can go on for eternity really until the sun swallows the Earth 5 billion years from now and our scattered remains go back to the vast universe where our molecules first formed.
Do you have a different answer that might help the questioner? How would you answer this question?