Why did all of my items disappear?

This has happened to me. When you open a world in 1.8 (or any post-1.7 snapshot), it changes the way the data is stored (tile entities, blocks and inventory data). This means, for example (using Torches): 50 becomes minecraft:torch.

When you open the world back in 1.7, Minecraft goes "What the heck are these?" (minecraft:torch) and therefore deletes it, as it is not recognized as a block/item/whatever. From there, you would find blocks such as signs/chests (item with TileEntity data) and items in your inventory missing.

The only way you can get your world back is to restore a backup (if you have one), like I did.
If you don't have one, then you'll have to start all over again, as there is no going back once you've opened the save. But if you have a windows 7 profesional you can restore an older version of the file.

This happens when 1 of 3 things happen:

1. Downloading a Mod and then removing it. Items from the mod including tables/chairs (or whatever the mod is about) will dissappear allong with some items.
2. Going from a newer version to an older version. This corrupts the file because the newer items no longer exist and are therefore nulled.
3. Going from an older modded game to a newer modded game. The mods are out of date so the items will disappear.