my items disapeared when i switched from 1.8 to 1.12

when i switched from to 1.12.1 to 1.8 my items wernt there and when i switched back they still wernt there and i had 2 stacks of gold 3 stacks of iron and 36 diamond an efficiency 4 diamond pickaxe 3 ender pearls 14 obsidian a full iron set and in my auto fishing farm i had 3 stacks of fish and 10 saddles