Maybe a quick check suggested by the principal themselves with various steps
http://knowledge.autodesk.com/suppor...orruption.html
Otherwise, try using DWG Recovery Toolbox s/w just to make the file legitimacy for recovery chances of prev copies. http://www.oemailrecovery.com/dwg_recovery.html
Hopefully from Autocad autosave by default, it should have saved dwg file automatically with ac$ as default extension, and if we can just find the *.ac$ file and change ac$ to dwg. There may be slight chance. The AutoCAD's autosave file is saved in this folder: C:\Users\[User Name]\appdata\local\temp\. typically.
Look at it this resource. Perhaps you will find there more useful information.
http://www.instructables.com/answers...tInvalid-File/
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