Please start AutoCAD® and type CUI on the command line so that Customize User Interface dialog box is displayed. In the top left section, expand Partial Customization Files, and check if SIGMA is on the list and if it is marked as Unresolved.
If you have SIGMA on the list and it is marked as Unresolved, please right click on it and unload it. Click on OK to close CUI dialog box, and then close AutoCAD®. Use SIGMA desktop shortcut to start it in AutoCAD®, and confirm that this time you can see SIGMA toolbar or ribbon.