Probably a Windows update messed it up.
You might try a repair, restore to defaults or a remove and reinstall of AutoCAD or get a newer version designed to run with Windows 10.
That's called the Quick Access Toolbar (QAT) if that helps.
What are your computer specifications?
What Windows version?
What happened that made it disappear?
If I recall correctly, this was an issue with running older AutoCAD versions on Windows 10 or newer, I believe AutoCAD 2016 was first for Windows 10.