Ribbon or toolbars are missing, blank, or disappear in …
https://knowledge.autodesk.com/support/autocad/troubleshooting/caas/sfdcarticles/sfdcarticles/Ribbon-and-toolbars-disappear.html
Toolbars, palettes, or ribbon commands are not added to the workspace Enter CUI. Select the specific workspace. Click "Customize Workspace" at the top right. Switch to the specific missing toolbar, palette, or ribbon command on the left side and check a hook for each. Click the button "Done". Close ...
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