Visit following
community:- http://www.filerepairforum.com/forum/cad-tools/cad-tools-aa/dwg/444-%E2%80%8Berror-cannot-open-file-open-cad-drawing-dwg-format
– here you may find out something interesting concerning your issue

If the drawing
file (DWG) will not open at all:

Your drawing may
show the message “Drawing file is not valid,” display a message about
errors being found, hang or crash during the open process, or simply may not
appear in AutoCAD after you have selected to open it (i.e., you open it but all
you ever see is the default “Drawing1.dwg” or another file that you
currently have open).

Use the following
article if you receive the “Drawing file is not valid” error when
opening the DWG: Error: “Drawing file is not valid” when opening a
file in AutoCAD.

You might have
leftover .dwl or dwl2 files in the folder directory. These are two hidden files
that have the same name as the .dwg file but with the extensions .dwl and
.dwl2. In the Windows Control Panel, go to Folder Options. On the View tab,
turn on the option to show hidden files. You can then go back to the folder
directory and delete the .dwl and .dwl2 files if they are found alongside your
DWG file. Attempt to open the DWG again.