If you are using a vertical to AutoCAD like Architecture, MEP, Mechanical or Civil, and would like to be able to get back to the classic interface such as the menus or toolbars like you could back in 2009, the safest way to do this is within the Customize User Interface (CUI)
From within your software type CUI at the command line. This will bring up the Customize User Interface editor. Select on the Transfer tab and notice that you have two panels that say "Customization in Main File" on the left and "Customization in New File" on the right.
You will be working on the right panel and copying items from right to left.
From the Right side click on the Open icon and navigate to your acad.cuix and click the open button.
The path to the .cuix file may vary for different OS or version of your software, a quick way to find that path would be to go into options within your software, you can do this by typing OPTIONS at the command line. Go to the Files tab and expand "Customization Files" you will then expand "Main Customization File", this will be where your .cuix files are located. View this photo Click here to read more about .cuix files.
Once you load the acad.cuix will now see on the right panel, all the AutoCAD workspaces. Expand Menus and using your shift key, select all the menus and drag and drop them into you menus on the left side. This will add those menus to your "Main Customization File". If you want to transfer any toolbars you will do the same steps by expanding Toolbars on the right, selecting the toolbars you want and drag and drop onto the toolbars on the left.
When you are finished click on the Apply button and click on the Customize tab to continue.
Within the "Customization in All Files" panel, click on the workspace that is set to Default (current) and click on the Customize Workspace button in the panel on the right.
You will now see back on the left side, all the items below the Workspaces will have check boxes. You will expand Menus and check all the menus you would like to show on your interface. When you are done selecting all that you want click the Done button from the "Workspace Contents" panel.
Click the Apply and OK button to get out of the Customize User Interface. You will need to verify that your workspace is currently set to save any changes you make within the Workspace Settings. You can verify this by either typing WSSETTINGS at the command line or click on the Gear icon in the lower right side of the interface.
Inside the Workspace Settings dialog box make sure the "When Switching Workspaces" is set to Automatically save workspace changes. Click the OK button to close the Workspace Settings dialog box.
In order for you to view your menus and not your ribbon you will need to type MENUBAR and set it to 1then type RIBBONCLOSE to turn off the ribbon.