Possibly the DWG file is a newer version of AutoCAD. I believe ArcMap can read up to AutoCAD 2013. One of the first things I do after adding a CAD file to the TOC of ArcMap is open the attribute table. If the table is empty than ArcMap cannot read the file. Also, some AutoCAD addons such as Civil Design can create objects that ArcMap cannot read.
If the file is not too big, you can check AutoCAD version by opening the DWG file in a text editor such as Notepad or Notepad Plus. Observe the first line and look for text such as AC1024.
Determine DWG File Version
AC1027 - AutoCAD 2013/2014
AC1024 - AutoCAD 2010/2011/2012
AC1021 - AutoCAD 2007/2008/2009
AC1018 - AutoCAD 2004/2005/2006
AC1015 - AutoCAD 2000/2000i/2002
AC1014 - Release 14
AC1012 - Release 13
AC1009 - Release 11/12
AC1006 - Release 10
AC1004 - Release 9