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):
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.