Open .dwg file
Community of enthusiasts must aid you- http://www.filerepairforum.com/forum/cad-tools/cad-tools-aa/dwg/444-%E2%80%8Berror-cannot-open-file-open-cad-drawing-dwg-format
Your best protection against this disaster is the AUTOMATIC SAVE feature. The interval between Automatic Saves is set on the OPEN and SAVE of the OPTIONS command.
So how do you recover an Automatic save file? When you open AutoCAD after a crash the DRAWING RECOVERY MANAGER should display. If not, type DRAWINGRECOVERY and press ENTER.
Backup files (BAK) and Automatic Save Files (SV$) that were found are displayed in the DRAWING RECOVERY MANAGER and can be reviewed and selectively recovered.
Otherwise apply DWG Viewer Tool
How it works, see:- http://www.dwg.viewertool.com/