If you unfortunately delete your drawings and still unlucky enough... because then you give an 's' (save) command to your CAD, don't panic! Do not close the window. Here the steps:
1. select all the latest drawing that you already have now. (ctrl A)
2. move (m) to the empty area in the same window.
3. protect it by (ctrl-shift-c).
4. undo till you get the previous session that you want.
5. save as file with another name.
5. (ctrl-shift-c) again to show the latest drawing, so you have two different versions now!
But, if you already close the window, because you DO panic, there are several choices:
The easy one, you can find BAK version of the file (this is the back up file from the AutoCAD) and change the extension from .bak to .dwg. The steps are:
1. open windows explorer.
2. fine the file name in BAK version.
3. rename, replace '.bak' with '.dwg'
4. open the file, save it or you can continue drawing from this back up.
When you cannot change the extension, or you cannot see the extension, do this:
1. in your windows explorer click 'tools' (or 'organize') then 'folder options'
2. click the 'view' tab.
3. uncheck 'hide extensions for known file types.'
4. rename, replace '.bak' with '.dwg'
5. open the file, save it or you can continue drawing from this back up.
If you are using Windows 7, the explorer provided a facility to restore the previous sessions (and it works to many kind of files). What you have to do is only right-click on the file name and click 'restore the previous version' or something sounds like that. But it only works if you already turn on the systems and files restore program. Use help from your explorer for the complete instructions.
Okay, be careful every time Pals! Enjoy your work :D