Welcome to the Autodesk/AutoCAD forums !
- If you have "only" an AutoCAD, it's not the right Forum !
If you have an AutoCAD MAP, it's OK !
A few suggestions ...
- Please open your DWG in RECOVERY Mode
and Re-Save it ...
- If AutoCAD 2013, have you installed the SP 2 ?
- If AutoCAD MAP 2013, have you installed the SP 2.1 ?
You may have to take it a step further:
Once you open with Recovery, if you cannot save the file still, try "Send" -> "eTransmit"
Save it in the same folder as your dwg. Extract the resulting zip file and open the dwg. This usually works for me and still keeps and FDO resources you have connected.
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