I have this one minecraft world that was on survival that I always played on. Then one day, I played on a snapshot; I opened my castle world on accident, but now all my chests are empty, and paintings and item frames are on the floor.
My animals are still there, but I have no items at all, even my armor is gone.
I tried replacing the world with a back up copy, but its the same issue.
Best Answer
There's a bug report on this issue on Mojang's bug tracker.
While the issue is confirmed, it was closed as an invalid bug, since it's an effect of downgrading versions. To quote:
The specific issue you are facing has come up in Snapshot 14w03a, specifically
Since you upgraded from a 1.7.9 to 14w21a (presumably), this change has taken effect. All items in the game save were converted to the new NBT format on world load. 1.7 doesn't know these weird items called "stone", "dirt" or "diamond_sword", it knows only "1", "3" and "276". The game therefore deletes the items, as Minecraft always does with unknown items.
As to why your backup doesn't work, I have no idea. I suggest you try again, and be careful not to open the world in a 1.8 snapshot if you don't plan on playing in that version. If your backup has been run in a 1.8 snapshot already, continue playing in 14w21b and don't downgrade to 1.7.
To prevent these problems in the future, you should set a different save directory in the profile for the snapshot (see my answer here) so that your regular world doesn't show up at all.