You probbably Downloaded a Mod and messed it up OR if you didnt you can fix it by (even if u did a mod this is the way) Open your .Minecraft Folder go to bin and Delete the version Folder open minecraft once again and your good to go
We lost:
22 diamonds
5 stacks of coal
7 stacks of redstone
1 Chest full of bread
14 stacks of red mushrooms
2 stacks of brown mushrooms
1 stack of iron
34 Gold ingots
1 stack of logs
1 chest full of cobble
12 stacks of dirt
and many other things