You may try:
1/ Reset AutoCAD - at the command prompt type OPTIONS and hit Return
In the "Application preferences" window go to Application tab, then on the right side pick on "Reset Application Options..." button.
If this won't help, then:
2/ Try Clean Uninstall:
https://knowledge.autodesk.com/support/autocad-for-mac/learn-explore/caas/sfdcarticles/sfdcarticles/...
then, install AutoCAD again using installer from here:
https://knowledge.autodesk.com/support/autocad-for-mac/downloads/caas/downloads/content/autocad-2013...

Keep in mind that in both cases you will loose all your customizations.