I have this problem, i was making a draw but never save this with a name, suddenly the autocad show me a Fatal error and close it, I was trying to found this file because the autocad was autosaving the file , but only found a drawing file with  the extension .DWL, I tried to change this to .DWG extension using MS DOS but this system show me  that it is no possible, I will appreciate if somebody can help me with this.

Look for a file with an .sv$ extension. This will be your autosave file that should have been saved when Autocad crashed. If you find it, rename it with the .dwg extension. If you can't find a file with that extension, you will have to start your drawing over again. In the future, SAVE YOUR WORK. Don't rely on Autocad to do it for you.

Do you have AutoCAD set up to create BAK files too?