All minecraft items disappeared?

Ok, so here is the story:
I couldn't wait for the new 1.8 update, so I decided to make a world using the latest snapshot (14w21b). I got pretty far in the world and got a lot of chests filled up. Yesterday, I was playing with a friend on multiplayer (Version 1.7.9) and she was doing something so I decided to play on my single player world for a while. I logged out, still on version 1.7.9, and logged into my practically 1.8 world with an earlier version installed. I immediately noticed that I had no items in my inventory, and all of my armor was gone. Upon further inspection, every item from my chests and furnaces had disappeared as well. I assumed it was because I was on a non-updated version, logged out, got back onto the 1.8 snapshot, and logged back into the world. Still, everything is gone. Honestly, I don't care if I can't get that stuff back, it's just that I spent a very long time building the house on that world. I just want to know if I need to give up on it or not because I won't start a new world until I know for sure I can't fix the problem. I can't go in creative and give myself items (no cheat mode). For clarity, all of my accumulated items are gone, but the blocks I placed are not.