World got overwritten by new world

I loaded up Minecraft, and wanted to work on an older world. I had to resync the world for some reason and the mashup pack. I use the Halo Texture pack, but I knew it was already installed beforehand. But for some reason, the world was completely overwritten, and I was put into a new world with the same name. I would like to know how to restore my older version, as I put a lot of time into the world.