Minecraft wiki entry for Recover_corrupted_saved_world_data, associated text: This can easily be done by restoring and "Re-creating" a backup of LostWorld. If no backup exists, you must manually create a new world. Make sure seed and map generator settings match those of LostWorld to avoid chunk error.
The inventory (i.e. the things you were carrying) from BlankWorld world will be used. Since an inventory in a new world is empty, it will also be empty in the restored world. You can fake a restored inventory by making BlankWorld a Creative world and putting everything you need into your inventory before closing the game. Don't forget your armor and any enchants on your armor and tools.
If you're not sure what the seed of LostWorld is and you need to find it, you can do so by accessing your level.dat file using a programme like NBTExplorer. Here's a step-by-step guide on how to find your seed using NBTExplorer.