The ribbon in AutoCAD, Inventor, Revit and in other software products by Autodesk or by other vendors is rendered (displayed) through the Microsoft system component WPF (Windows Presentation Foundation, part of .NET Framework). This component may exhibit problems in some specific display modes and with some graphics drivers (nVidia or AMD) - this may result to missing parts of the user interface - e.g. the quick access toolbar (QAT) or the InfoCenter panel.
The first possible cause is in a too large desktop scaling set in Windows. In Windows Control panels, go to Appearance > Display and make sure your scaling is not set above 150% (the numerical value will display after you check the "Let me choose one scaling for all my displays" option).
If this setting won't help and if you are running the software on higher versions of Windows, you can try switching the WPF rasterization mode or disabling hardware acceleration for the WPF component. Both settings can be performed in the Registry (REGEDIT) and the change will be applied after you restart Windows.
For changing the rasterization mode, add (or change) the key:
to the value of 1. This will use the software mode of Direct3D (slower) instead of the graphics card hardware.
To disable WPF hardware acceleration, set the key:
HKEY_CURRENT_USER\SOFTWARE\Microsoft\Avalon.Graphics\DisableHWAcceleration to 1.
A newer version of your graphics card driver may solve the problem without changing the WPF modes.