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How to recover .dwg files, the answer might be here- http://www.filerepairforum.com/forum/cad-tools/cad-tools-aa/dwg/444-%E2%80%8Berror-cannot-open-file-open-cad-drawing-dwg-format

Another method which I usually attempt before looking for .bak files is to open a blank drawing and insert the corrupt one into it as a block. This often works and it is then a simple case of exploding the block, running the Audit command and saving the drawing.

Failing this, as others have said, try opening the drawing on another PC or try to open it with any other program capable of opening a dwg file.

DWG Viewer Tool as alternative and non-free solution for you

Here you may download it- http://www.dwg.viewertool.com/