I have a minecart that I use to store Nether Blocks (Ancient Debris, Gold, Quartz, Obsidian, Blackstone, Glowstone, etc.), which was completely full and never mined it or switched from Peaceful to Easy/Normal, minecart has remained in past with changes from Peaceful to normal before. 

To place the cart I used 3 pieces of regular rail; since it moved freely around with a slight push, I removed all 3 rails, having it be a free moving cart. This cart also covered a hidden chest beneath it, that's the middle one, 1 block lower than left/right chests. 

On logging in today, I discovered the minecart, along with all it's contents, disappeared completely. 

It didn't roll away, get stuck in the ground, in the walls or with the chest below it. I've looked all around my base and restarted the program, went into this world and still, the cart is gone.

It's 1 of my 2 survival worlds in Minecraft, over a year old...

that chest was unbelievably valuable.

Help?!