It may be a problem with your system variables. There were many times I would get a call from frustrated AutoCAD customers. Usually the issue dealt with one or more system variables that had inadvertently changed from their default setting. The hard part for both of us was to try to figure out which variable needed to be fixed and what setting it was originally on.
I am please to see that in the latest versions of AutoCAD, there is a command called SysvarMonitor!
In the dialog box we find some of the common system variables that have been known to cause frustration if or when they gets changed. You may also add more variables to the list, by clicking on the "Edit List" button on the upper right. If one of these monitored system variables changes and the two check boxes are selected above, then you will receive notification when it happens. You will see an Icon appear in the system tray and balloon notification. This is what it looks like.