Most of the time a clean Uninstall and Reinstall will resolve this issue:
Open your Control Panel
Click Add / Remove Programs
Double Click on AutoCAD
Select Uninstall from your options
Now you can reinstall AutoCAD from scratch.
Note: If this does not resolve the issue you might need to uninstall again and delete the following files before the reinstallation:
C:ProgramDataFLEXnetadskflex_00691b00_tsf.data