How to separate line art from the white in GIMP?

Gimp didn't save after I edited a picture, how do I get the old one back?

• I've working on this picture for days, and I always save after I finish what-ever part of the picture I'm working on. My comp glitched right as I was deleting a layer and then saving so it ended up deleting 4 layers (I have no idea how) and then saving. It wasn't a big deal at the time so I just undid the delete of the 3 layers I wanted to keep and kept working. I finished a couple more parts and saved a couple more times, but when I opened up Gimp this morning the last time it saved was after deleting the 4 layers. I've never had this problem, even when my comp has glitched like this before. The layers were the background, an under sketch/skeleton, line art, and a duplicate layer I was testing something on. The main thing I need back is the sketches; I can always redo background and line art (which I wasn't finished with anyway), but the sketches were what took up most of my time. I don't have the time or patience to work on this project for another week just to get back to where I had been last night. Please, I'm desperate! If someone can help me I'll be forever grateful...

• Answer:

  Perhaps it saved as a different type of file rather than JPG. I've used GIMP and thought things have been unsaved even though I definitely saved them and then I notice them saved as bitmap or something else.