I had a Hardcore World for months, and I had some very beautiful builds: a fully lit fortress and an End colony, built on manually constructed islands. I quitted the game on the Living Quarters island. When I re-entered the world, I fell through Stone right into the Void! I tried to teleport, but I missed slightly. Because of this, I lost everything, including that fully enchanted Diamond Armour.

Why did this horrible thing happen?

P.S.: Today is my birthday, and it feels pretty bad for it to happen today.