I just want to re-emphasize ctrl + z here. Stuff like this happens to all of us. But the first thing you should try and do is "undo" to see if it will come back, you might have to hit it a couple of times if a couple of things were involved in the process between what you had and where you ended up.

Also, while auto-save is a nice feature, it's a good habit to be hitting ctrl + s every once in awhile, every couple of sentences or so, and just manually saving your document as often as possible. Never know when something might crash.