Are you sure you didn't save it in some obscure location by accident? That has happened to me before. Check your temp docs and all your folders!

One way to eliminate this from happening as well is to save that doc as soon as you open it as a file. Then every line or two you type, hold CTRL and type 's' and you automatically save your work as you go. I learned that after losing a couple chapters myself, so I sympathize.

*hugs*