or: Drawing file not valid, not a valid file
You are probably trying to open an AutoCAD 2009/2008/2007 drawing in AutoCAD 2006, an AutoCAD 2011 drawing in AutoCAD 2004, or an AutoCAD 2004 DWG file in AutoCAD 2000/2000i/2002 (generally: a newer DWG format in an older AutoCAD version). Or your DWG file is corrupted.
In the source AutoCAD version, re-save your file into a compatible (older) DWG file format (using the SAVEAS command or OPTIONS > Open&Save > Save As), or upgrade to the current version of your Autodesk product (e.g. 2015).
You can also use the offical DWG-version-converter (free) - "Autodesk DWG TrueView 2015" (see www.cadstudio.cz/download).