Every once and a while your AutoCAD session will crash and burn.  You might be ale to answer yes to the do you want to save your changes dialogue box if the crashing mechanism wasn't so bad, but most of the time AutoCAD just disappears.  Here are a few tips for dealing whit this scenario.

The best safeguard to this is to simply save your drawing using the PostalCAD keyboard shortcut QS, for Quick Save, or use the AutoCAD one, CTRL+S.  A rule of thumb is that after every 5 commands, use the save command.  The more you save, the less frustrating the recreation becomes.

Another safeguard mechanism is the Drawing Recovery Manger that will appear after the crash.  This is a very nice utility that lists all of the drawings you had open and the .bak files to open just in case the .dwg had gotten corrupted in the crash.  Just double click on the drawings you want to open and check.  Close the palette when you're done.  Just that simple.