Ribbon or toolbars are missing, blank, or disappear in …
https://knowledge.autodesk.com/support/autocad/troubleshooting/caas/sfdcarticles/sfdcarticles/Ribbon-and-toolbars-disappear.html
On the command line, type MENUBAR and set the value to 1. Click the Tools > Toolbars menu. Select the desired toolbars to turn them on. Select or reset the current workspace On the Status bar, click the drop-down arrow next to the Workspace gear icon. Select a workspace or switch to a different one. If needed, switch back to the desired workspace.
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